Optical constructions incorporating a light guide and low reflective index films

ABSTRACT

Optical constructions use a low index of refraction layer disposed between a low absorption layer and a high absorption layer to increase confinement of light to the low absorption region of the optical constructions. Low index layers can be used in optical constructions that have multi-tiered light confinement. In these constructions, a first tier of reflection is provided when light is reflected at the surface of a low index optical film which is disposed directly or indirectly on a light guide. A second tier of reflection occurs at the surface of a light redirecting film having appropriately oriented refractive structures.

CROSS-REFERENCE TO RELATED CASES

This application is a continuation of U.S. application Ser. No.13/511,381 filed on May 23, 2012, which claims priority toPCT/US2010/058526, filed on Dec. 1, 2010, which claims the benefit ofU.S. Provisional Application No. 61/267,631, filed on Dec. 8, 2009, thedisclosures of which are incorporated by reference in their entiretyherein.

RELATED APPLICATIONS

This application is related to the following U.S. patent applicationswhich are incorporated herein in their entireties by reference: “OpticalFilm” (Attorney Docket No. 65062US002) filed on Apr. 15, 2009 and havingSer. No. 61/169,466; “Optical Construction and Display SystemIncorporating Same” (Attorney Docket No. 65354US002) filed on Apr. 15,2009 and having Ser. No. 61/169,521; “Retroreflecting OpticalConstruction” (Attorney Docket No. 65355US002) filed on Apr. 15, 2009and having Ser. No. 61/169,532; “Optical Film for Preventing OpticalCoupling” (Attorney Docket No. 65356US002) filed on Apr. 15, 2009 andhaving Ser. No. 61/169,549; “Backlight and Display System IncorporatingSame” (Attorney Docket No. 65357US002) filed on Apr. 15, 2009 and havingSer. No. 61/169,555; “Process and Apparatus for Coating with ReducedDefects” (Attorney Docket No. 65185US002) filed on Apr. 15, 2009 andhaving Ser. No. 61/169,427; “Process and Apparatus for A NanovoidedArticle” (Attorney Docket No. 65046US002) filed on Apr. 15, 2009 andhaving Ser. No. 61/169,429; and “Optical Construction and Method ofMaking the Same” (Attorney Docket No. 65619US002) filed on Oct. 22, 2009and having Ser. No. 61/254,243.

This application is further related to the following U.S. patentapplications, all filed on Oct. 24, 2009 and which are incorporated intheir entireties by reference: “Light Source and Display SystemIncorporating Same” (Attorney Docket No. 65782US002) and having Ser. No.61/254,672; “Gradient Low Index Article and Method” (Attorney Docket No.65716US002) and having Ser. No. 61/254,673; “Process for GradientNanovoided Article” (Attorney Docket No. 65766US002) and having Ser. No.61/254,674; “Immersed Reflective Polarizer with High Off-AxisReflectivity” (Attorney Docket No. 65809US002) and having Ser. No.61/254,691; “Immersed Reflective Polarizer With Angular Confinement inSelected Planes of Incidence” (Attorney Docket No. 65900US002) andhaving Ser. No. 61/254,692; and “Voided Diffuser” (Attorney Docket No.65822US002) and having Ser. No. 61/254,676.

FIELD OF THE INVENTION

This invention generally relates to optical constructions and to lightsources and/or display systems that incorporate such opticalconstructions.

BACKGROUND

Optical displays, such as liquid crystal displays (LCDs), are becomingincreasingly commonplace, finding use in many applications such asmobile telephones, hand-held computer devices ranging from personaldigital assistants (PDAs) to electronic games, to larger devices such aslaptop computers, LCD monitors and television screens. LCDs typicallyinclude one or more light management films to improve displayperformance, including output luminance, illumination uniformity,viewing angle, and overall system efficiency. Exemplary light managementfilms include prismatically structured films, reflective polarizers,absorbing polarizers, and diffuser films.

Major trends in the display industry include reducing the cost of thelight source, reducing the number of components in the light source, andmaking light sources thinner and more efficient.

SUMMARY

Some embodiments describe an optical construction that includes a lowindex layer having an index of refraction, Nuli, where Nuli is notgreater than about 1.35. The optical construction also includes a highabsorption layer and a light redirecting film. Substantial portions ofeach of two neighboring films in the optical construction are inphysical contact with each other.

Some embodiments illustrate an optical construction including a lightguide having first surface and a second surface comprising a major lightexit surface of the light guide. The optical construction also includeslight redirecting film. A low index layer is disposed between the lightguide and the light redirecting film, the low index layer having anindex of refraction not greater than 1.35. The low index layer may beattached to the second surface of the light guide and to the lightredirecting film.

Another embodiment illustrates an optical construction including atleast one light guide having an index of refraction N1 and a low indexlayer having an index of refraction, Nuli, where Nuli is less than N1.The optical construction includes a light redirecting film, whereinsubstantial portions of each of two neighboring films in the opticalconstruction are in physical contact with each other.

Yet another embodiment illustrates an optical construction that includesa light guide, having first and second major surfaces and an index ofrefraction N1 and a low index layer having first and second majorsurfaces, the low index layer having an index of refraction, Nuli, whereNuli is less than N1, wherein a substantial portion of the first majorsurface of the low index layer is in physical contact with the secondmajor surface of the light guide. The optical construction also includesa high absorption layer having a first major surface and a second majorsurface, wherein a substantial portion of the first major surface of thehigh absorption layer is in physical contact with the second majorsurface of the low index layer. The optical construction furtherincludes a prism film, having a first major surface and a second majorsurface, wherein the first major surface comprising linear prisms and asubstantial portion of the first major surface of the light re-directingfilm is in physical contact with the second major surface of the highabsorption layer. The low index layer reflects light exiting from thelight guide at a first set of exit angles and the light re-directingfilm is configured to reflect light exiting from the light guide at asecond set of exit angles.

BRIEF DESCRIPTION OF DRAWINGS

Various embodiments of the invention may be more completely understoodand appreciated in consideration of the following detailed descriptionof various embodiments of the invention in connection with theaccompanying drawings, in which:

FIGS. 1A-1J are schematic side views of optical constructionsincorporating low index refractive layers in accordance with embodimentsof the invention;

FIG. 2 is a diagram illustrating the operation of two tiered reflectionin an optical construction in accordance with embodiments of theinvention;

FIG. 3A is a schematic side view of a low index optical film inaccordance with embodiments of the invention;

FIGS. 3B-3G are schematic side views of gradient low index optical filmsin accordance with embodiments of the invention;

FIG. 4A is a diagram of interleaved optical constructions in accordancewith embodiments of the invention;

FIG. 4B is a diagram of tiled optical constructions in accordance withembodiments of the invention;

FIG. 4C is a diagram of a light guide having a number of channels withLED light sources recessed therein;

FIG. 4D shows a light guide having surface features that redirect lightinjected from a light guide surface to angles within the TIR range ofthe light guide;

FIGS. 5-7 are optical micrographs of porous low index optical films inaccordance with embodiments of the invention;

FIG. 8A is a cross-sectional micrograph of a gradient low index opticalfilm; FIG. 8B is a higher magnification of the micrograph in FIG. 8A;

FIG. 9A is a schematic side view of the optical construction tested inExample 1;

FIG. 9B is a photograph taken of an edge of the optical construction ofExample 1 showing the hall of mirrors effect;

FIG. 10A is a schematic side view of the optical construction tested inExample 2a;

FIG. 10B is a grayscale image of the measured luminance of the opticalconstruction of Example 2a as a function of viewing angle;

FIG. 10C is a graph of the measured luminance of the opticalconstruction of Example 2a as a function of viewing angle along thehorizontal direction;

FIG. 10D is a graph of the measured luminance of the opticalconstruction of Example 2a as a function of viewing angle along thevertical direction;

FIG. 11A is a schematic side view of the optical construction of Example2b;

FIG. 11B is a grayscale image of the measured luminance of the opticalconstruction of Example 2b as a function of viewing angle;

FIG. 11C is a graph of the measured luminance of the opticalconstruction of Example 2b as a function of viewing angle along thehorizontal direction;

FIG. 11D is a graph of the measured luminance of the opticalconstruction of Example 2b as a function of viewing angle along thevertical direction;

FIG. 12A is a schematic side view of the optical construction of Example3;

FIG. 12B is a photograph taken of an edge of the optical construction ofExample 3 showing the hall of mirrors effect;

FIG. 13A is a schematic side view of the optical construction of Example4;

FIG. 13B is a grayscale image of the measured luminance of the opticalconstruction of Example 4 as a function of viewing angle;

FIG. 13C is a graph of the measured luminance of the opticalconstruction of Example 4 as a function of viewing angle along thehorizontal direction;

FIG. 13D is a graph of the measured luminance of the opticalconstruction of Example 4 as a function of viewing angle along thevertical direction;

FIG. 14 shows the construction and ray tracing modeling result forExample 5;

FIG. 15 shows the construction and ray tracing modeling result forExample 6;

FIG. 16 shows the construction and ray tracing modeling result forExample 7;

FIG. 17 shows the construction and ray tracing modeling result forExample 8;

FIG. 18 shows the construction and ray tracing modeling result forExample 9;

FIG. 19 shows the construction and ray tracing modeling result forExample 10;

FIG. 20A is a graph of the amount of light guided in an opticalconstruction with a low index film but without a light redirectinglayer;

FIG. 20B is a graph of the amount of light guided in an opticalconstruction that has a low index film and a light redirecting layeroriented with prisms perpendicular to the direction of lightpropagation;

FIG. 20C is a graph of the amount of light guided in an opticalconstruction that has a low index film and a light redirecting layeroriented with prisms parallel to the direction of light propagation;

FIG. 21A is a schematic side view of the optical construction tested inExample 12;

FIG. 21B is a graph showing the amount of light being absorbed by thehigh absorption region for 32″ and 52″ backlights plotted against therefractive index of the low index layer.

DETAILED DESCRIPTION

Various embodiments of the invention are directed to opticalconstructions that include a low index of refraction optical film whichhas the ability to support total internal reflection at the exit surfaceof a low absorption optical layer such as a light guide. The low indexoptical film may be disposed between a light guide and a lightredirecting film to provide two tiers of internal reflection. The lowindex optical film may be disposed between a low absorption layer and ahigh absorption layer, increasing the optical confinement of the lightwithin the low absorption layer.

Incorporation of the disclosed optical constructions into variousoptical or display systems, for example, a liquid crystal displaysystem, may improve system durability, reduce manufacturing cost,increase the efficiency, and/or reduce the overall thickness of thesystem. The optical constructions discussed herein may be used indisplays, hand held applications, such as cell phones and personal dataassistants (PDAs), notebook and desktop computer monitors, signs,luminaires, projection systems, and/or other applications.

Embodiments described herein illustrate the use of one or more opticalfilms having a low index of refraction to reduce exposure of light tohigh absorption materials and/or to reduce premature light extractionusing multiple tiers of reflection.

In some configurations, the low index film is disposed between a highabsorption layer and a low absorption layer, e.g., a light guide, toincrease confinement of light within the low absorption material andreduce exposure of most of the light to the higher absorption layer.During light transport from the edge of the light guide, the light isprimarily exposed to the low absorption material of the light guide andexposure to the high absorption material does not occur until the lightis extracted over a short path length at an angle close to normal to thesurface. Increasing the confinement of light to low absorption materialsreduces the exposure of light to high absorption materials, therebyimproving system efficiency.

In some configurations, a film having a low index of refraction isdisposed on a major exit surface of a light guide to provide a firsttier of total internal reflection (TIR) within the light guide. In theseconfigurations, some of the light in the light guide may neverthelessescape (leak) from the light guide before extraction, e.g., extractionby the light guide extraction features. The degree of light leakage isdependent on the difference between the refractive index of the lightguide and the refractive index of the low index film. A higher indexguide collimates (by refraction) edge injected light more than a lowerindex guide due to Snell's law. If light is insufficiently collimated,it will not be within the TIR confinement range, and will leak out themajor surface of the guide.

Some optical constructions described herein illustrate a multi-tieredTIR approach that enhances the amount of light contained within theoptical construction until it is extracted by the light guide extractionfeatures. As discussed in more detail below, a first tier of TIR isprovided when light is reflected at the surface of a low index opticalfilm which is disposed directly or indirectly on a light guide. A secondtier of TIR occurs at the surface of a light redirecting film havingappropriately oriented refractive structures. Additional tiers of TIRare also possible by incorporating additional layers.

FIG. 1A illustrates a cross section diagram of an optical construction100A in accordance with embodiments of the invention. Opticalconstruction 100A includes a low absorption layer 101 having absorptionA1 and an index of refraction N1. The low absorption layer 101 maycomprise a light guide, for example. The optical construction 100Aincludes a high absorption layer 103. High absorption layer 103, whichmay comprise a polarizer, a structured film, and/or a light redirectingfilm, for example, has absorption A3 and index of refraction N3. Theabsorption of high absorption layer 103 is greater than the absorptionof low absorption layer 101. In some embodiments, the absorption of thehigh absorption layer 103 is about twice the absorption of the lowabsorption layer 101. The high absorption layer 103 may includeweathering absorbers for blue or UV, absorbing polarizers, reflectivepolarizers, phosphors, microreplication resins for structured surfaces,multilayer films, and/or other layers. Particularly for signs andgraphics applications the high absorption layer may include printinginks and paints which contain dyes, pigments, and scattering materialssuch as titanium indium oxide (TIO2).

Disposed between the low absorption layer 101 and high absorption layer103 is low index optical film 120. Low index optical film 120 hasabsorption A2 and an index of refraction N2, where N2<N1. In someembodiments, N2 is between about 1.10 and N1. In some embodiments, N2 isless than about 1.35. Substantial portions of two neighboring majorsurfaces in optical construction 100A are in physical contact with eachother. For example, substantial portions of neighboring major surfaces105 and 106 of respective neighboring layers 101 and 120 are in physicalcontact with each other. Additionally or alternatively, substantialportions of neighboring major surfaces 107 and 108 of respectiveneighboring layers 120 and 103 are in physical contact with each other.The phrases “substantial portions in physical contact” or “substantialphysical contact” means that at least 50%, or at least 60%, or at least70%, or at least 80%, or at least 90%, or at least 95% of theneighboring major surfaces are in physical contact with each other.

In some cases, low index optical film 120 is formed or coated directlyon the low absorption layer 101. In some cases, optical film 120 isformed or coated directly on the high absorption layer 103. In somecases, one or more layers, e.g., an adhesive layer, may be disposedbetween the low absorption layer 101 and the optical film 120, and/orbetween the high absorption layer 103 and the optical film 120. In theseconstructions, substantial portions of two neighboring major surfaces inthe optical construction are in physical contact with each other. Forexample, in an optical construction having an adhesive layer disposedbetween light guide 101 and low index optical film 120, substantialphysical contact occurs between a first major surface of the adhesivelayer and surface 105 of the light guide 101 and/or substantial physicalcontact occurs between a second major surface of the adhesive layer andsurface 106 of the low index optical film 120. In some cases a diffuserlayer can be disposed between low index layer 120 and high absorptionlayer 103. Diffuser layer can be any type of diffuser includingscattering particles in a matrix, or a diffuser layer comprising aplurality of voids such as described in co-pending patent applicationtitled “Optical Film” (Attorney Docket No. 65062US002) filed on Apr. 15,2009 and having Ser. No. 61/169,466.

FIG. 1B illustrates a cross sectional diagram of an optical construction100B in accordance with embodiments of the invention. The opticalconstruction 100B includes a light guide 110 and a low index opticalfilm 120 disposed on the light guide 110. A light redirecting film 130is disposed on the low index optical film 120. The light guide 110 hasan index of refraction of N1 and the low index optical film 120 has anindex of refraction of N2, wherein N2<N1, or N2 is less than 1.35, orless than 1.30, or less than 1.25, or less than 1.20, or less than 1.15or is between about 1.10 and N1, for example. Substantial portions oftwo neighboring major surfaces in optical construction 100B are inphysical contact with each other. For example, in one implementation,substantial portions of neighboring major surfaces 112 and 121 ofrespective neighboring layers 110 and 120 are in physical contact witheach other and/or substantial portions of neighboring major surfaces 122and 131 of respective neighboring layers 120 and 130 are in physicalcontact with each other.

In some cases, low index optical film 120 is formed or coated directlyon light guide 110. In some cases, low index optical film 120 is formedor coated directly on light redirecting film 130. In some cases, lightredirecting film 130 has an absorption that is greater than theabsorption of light guide 110 and an index of refraction between about1.5 and 1.8, for example.

In some implementations, there may be one or more additional layersdisposed between the light guide 110 and the low index optical film 120as shown by the cross sectional diagram of an optical construction 100Cin FIG. 1C. In this example, low index optical film 120 is attached tothe light guide 110 by adhesive layer 115. Low index optical film 120 isattached to the light redirecting film 130 by adhesive layer 125.Substantial portions of neighboring major surfaces the neighboringlayers in optical construction 100C are in physical contact with eachother.

In optical construction 100C, major surface 112 of light guide 110 is insubstantial physical contact with major surface 113 of adhesive layer115, major surface 114 of adhesive layer 115 is in substantial physicalcontact with major surface 121 of low index film 120, surface 122 of lowindex film 120 is in substantial physical contact with surface 123 ofadhesive layer 125, surface 124 of adhesive layer 125 is in substantialphysical contact with surface 131 of light redirecting film 130.

FIG. 1D illustrates an optical construction 100D that includes one ormore high absorption layers 160 disposed between low index optical film120 and light redirecting film 130. The high absorption layer 160 maycomprise, for example, an absorbing polarizer, a reflective polarizer,weathering absorbers for blue or UV light, phosphors, layers comprisingmicroreplication resins having structured surfaces. The high absorptionlayer 160 may also include printing inks and paints which contain dyes,pigments, and scattering materials such as TIO2. These types ofabsorbing layers are of particular importance in signs and graphicsapplications. Optical construction 100C may include an adhesive layerwith a removable backing layer (not shown) that is adhered to thesurface 121 of the low index layer 120, providing a light managementstack ready to be adhesively attached to a light guide after removingthe adhesive backing. In some cases, low index optical film 120 may beformed on the high absorption layer 160 and in some cases, the low indexoptical film 120 may be adhesively attached to the high absorption layer160.

FIG. 1E shows an optical construction 100E which includes the opticalconstruction 100D illustrated in FIG. 1D disposed on a light guide 110.The light guide 110 has absorption A1 and the high absorption layer 160has absorption A3, where A1 is less than A3. In some cases A3 is abouttwice A1.

If a reflective polarizer is used as the high absorption layer 160,reflective polarizer 160 substantially reflects light that has a firstpolarization state and substantially transmits light that has a secondpolarization state, where the two polarization states are mutuallyorthogonal. For example, the average reflectance of reflective polarizer160 in the visible for the polarization state that is substantiallyreflected by the reflective polarizer 160 is at least about 50%, or atleast about 60%, or at least about 70%, or at least about 80%, or atleast about 90%, or at least about 95%. As another example, the averagetransmittance of reflective polarizer 160 in the visible for thepolarization state that is substantially transmitted by the reflectivepolarizer 160 is at least about 50%, or at least about 60%, or at leastabout 70%, or at least about 80%, or at least about 90%, or at leastabout 95%, or at least about 97%, or at least about 98%, or at leastabout 99%. In some cases, reflective polarizer 160 substantiallyreflects light having a first linear polarization state (for example,along the x-direction) and substantially transmits light having a secondlinear polarization state (for example, along the y-direction).

Any suitable type of reflective polarizer may be used for reflectivepolarizer layer 160 such as, for example, a multilayer optical film(MOF) reflective polarizer, a diffusely reflective polarizing film(DRPF) having a continuous phase and a disperse phase, such as aVikuiti™ Diffuse Reflective Polarizer Film (“DRPF”) available from 3MCompany, St. Paul, Minn., a wire grid reflective polarizer described in,for example, U.S. Pat. No. 6,719,426, a fiber, blend, or cholestericreflective polarizer.

In some cases, reflective polarizer 160 can be or include an MOFreflective polarizer, formed of alternating layers of different polymermaterials, where one of the sets of alternating layers is formed of abirefringent material, where the refractive indices of the differentmaterials are matched for light polarized in one linear polarizationstate and unmatched for light in the orthogonal linear polarizationstate. In such cases, an incident light in the matched polarizationstate is substantially transmitted through reflective polarizer 160 andan incident light in the unmatched polarization state is substantiallyreflected by reflective polarizer layer 160. The MOF may comprise acollimating or non collimating polarizer and may be an extended bandpolarizer. In some cases, an MOF reflective polarizer 160 can include astack of inorganic dielectric layers.

As another example, reflective polarizer 160 can be or include apartially reflecting layer that has an intermediate on-axis averagereflectance in the pass state. For example, the partially reflectinglayer can have an on-axis average reflectance of at least about 90% forvisible light polarized along the x-direction, and an on-axis averagereflectance in a range from about 25% to about 90% for visible lightpolarized in a second plane along the y-direction. Such partiallyreflecting layers are described in, for example, U.S. Patent PublicationNo. 2008/064133, the disclosure of which is incorporated herein in itsentirety by reference.

In some cases, reflective polarizer 160 can be or include a circularreflective polarizer, where light circularly polarized in one sense,which may be the clockwise or counterclockwise sense (also referred toas right or left circular polarization), is preferentially transmittedand light polarized in the opposite sense is preferentially reflected.One type of circular polarizer includes a cholesteric liquid crystalpolarizer.

In some cases, reflective polarizer 160 can be a multilayer optical filmthat reflects or transmits light by optical interference, such as thosedescribed in Provisional U.S. Patent Application No. 61/116,132, filedNov. 19, 2009; Provisional U.S. Patent Application No. 61/116,291, filedNov. 19, 2008; Provisional U.S. Patent Application No. 61/116,294, filedNov. 19, 2008; Provisional U.S. Patent Application No. 61/116,295, filedNov. 19, 2008; and International Patent Application No. PCT/US2008/064115, filed May 19, 2008, claiming priority from Provisional U.S.Patent Application No. 60/939,085, filed May 20, 2007; U.S. ProvisionalPatent Application No. 61/254,672 entitled “Light Source and DisplaySystem Incorporating Same,” Attorney Docket No. 65782US002, filed Oct.24, 2009; U.S. Provisional Patent Application No. 61/254,673 entitled“Gradient Low Index Article and Method,” Attorney Docket No. 65716US002,filed Oct. 24, 2009; U.S. Provisional Patent Application No. 61/254,674entitled “Process for Gradient Nanovoided Article,” Attorney Docket No.65766US002, filed Oct. 24, 2009; U.S. Provisional Patent Application No.61/254,691 entitled “Immersed Reflective Polarizer with High Off-AxisReflectivity,” Attorney Docket No. 65809US002, filed Oct. 24, 2009; U.S.Provisional Patent application No. 61/254,692 entitled “ImmersedReflective Polarizer With Angular Confinement in Selected Planes ofIncidence,” (Attorney Docket No. 65900US002, filed Oct. 24, 2009; andU.S. Provisional Application No. 61/254,676 entitled “Voided Diffuser,”Attorney Docket No. 65822US002, filed Oct. 24, 2009, all incorporatedherein by reference in their entireties.

Substantial portions of neighboring major surfaces in opticalconstructions 100D and 100E are in physical contact with each other. Insome cases, low index optical film 120 is coated directly on the surface144 of high absorption layer 160 or low index optical film 120 can becoated or formed on the light guide 110.

There may be one or more additional layers disposed in between highabsorption layer 160 and low absorption optical film 120 and/or betweenthe light guide 110 and the low absorption optical film 120 ofconstruction 1E. For example, FIG. 1F is a schematic side-view of anoptical construction 100F that includes an optical adhesive layer 162disposed between optical film 120 and high absorption layer 160, e.g., apolarizer layer for adhering the optical film 120 to the polarizer layer160. FIG. 1F also illustrates an optical adhesive 166 disposed betweenthe light redirecting film 130 and the high absorption layer 160 and anoptical adhesive 115 disposed between the light guide 110 and the lowindex optical film 120. Substantial portions of neighboring majorsurfaces of respective neighboring layers of optical construction 100Fare in physical contact with each other. Not all the adhesive layers115, 162, and 166 need to be used for an optical construction.

Optical adhesive layers 115, 162, 166 (FIG. 1F) and 125 (FIG. 1C) caninclude any optical adhesive that may be desirable and/or available inan application. Exemplary optical adhesives include pressure sensitiveadhesives (PSAs), heat-sensitive adhesives, solvent-volatile adhesives,and UV-curable adhesives such as UV-curable optical adhesives availablefrom Norland Products, Inc. Exemplary PSAs include those based onnatural rubbers, synthetic rubbers, styrene block copolymers,(meth)acrylic block copolymers, polyvinyl ethers, polyolefins, andpoly(meth)acrylates. As used herein, (meth)acrylic (or acrylate) refersto both acrylic and methacrylic species. Other exemplary PSAs include(meth)acrylates, rubbers, thermoplastic elastomers, silicones,urethanes, and combinations thereof. In some cases, the PSA is based ona (meth)acrylic PSA or at least one poly(meth)acrylate. Exemplarysilicone PSAs include a polymer or gum and an optional tackifying resin.Other exemplary silicone PSAs include a polydiorganosiloxane polyoxamideand an optional tackifier. The adhesive may be or include a reusableand/or repositionable adhesive such as those described in, for example,U.S. Pat. No. 6,197,397; U.S. Patent Publication No. 2007/0000606; andPCT Publication No. WO 00/56556, the disclosures of which areincorporated herein in their entireties by reference. The phrases“reusable adhesive” or “repositionable adhesive” for adhering a film toa substrate mean an adhesive that (a) affords a temporary, secureattachment of the film to the substrate while affording convenient,manual removal of the film from the substrate without damaging thesubstrate or exhibiting excessive adhesive transfer from the film to thesubstrate, and (b) then affords subsequent reuse of the film on, forexample, another substrate. For example, if adhesive 115 is arepositionable adhesive, then the low index film 120 is repositionablebecause low index film 120 can be repositioned on the light guide 110.

An optical construction 100G is illustrated in FIG. 1G. Opticalconstruction 100G includes a light guide 110 having an optical film 120disposed on the light guide 110 either directly or indirectly throughone or more additional intervening layers (not shown in FIG. 1G).Substantial portions of neighboring major surfaces of respectiveneighboring layers of optical construction 100G are in physical contactwith each other. Light is generated by one or more light sources 192,which may be oriented near an input edge 173 of the light guide 110 asillustrated in FIG. 1G, or may be remote from the light guide 110 withlight delivered to the light guide 110 through a delivery means, such asa light fiber or hollow cavity. Light rays enter the light guide 110 atthe input edge 173 and are propagated by total internal reflection (TIR)along a light propagation direction 175. The input edge 173 may comprisea flat input edge, a structured surface, and/or may include a recess forthe light source.

The major surfaces of 111, 112 of light guide 110 may be substantiallyparallel as illustrated in FIG. 1G, or the light guide 110 may bewedge-shaped. The light guide 110 may be flat or curved.

A backlight may comprise one or more optical constructions such asoptical construction 100G, for example. The backlight 100G may providelight for a liquid crystal display (LCD) panel disposed above the lightredirecting film of optical construction 100G, for example.

A backlight may include one or more light guides and/or one or morelight sources and/or may include a control system that collectively orindividually controls the operation of the light sources and/or otherbacklight elements.

For example, a backlight may be a tiled system facilitating dynamicbacklighting that delivers improved contrast and energy efficiency.Tiled backlights may have overlapping light guide tiles. Each lightguide or groups of light guides may be paired with an individuallycontrollable light source. Diffusers can be used to blenddiscontinuities at the overlapping tile interfaces.

In another example, a backlight may comprise a field sequential systemin which the backlight rapidly pulses red, green, blue in sequence, andLCD pixel shutters open and close in sync with the backlight pulsation.The pixels that are open when a given color pulses from the backlightdepend on the image being displayed.

In yet another example, the backlight may be a zoned system. Zonedsystems selectively dim a portion of the backlight to provide both powersavings and improved contrast. This dimming is referred to as zoning,since the backlight is controlled in spatial zones rather than one largepanel of light. Zones can be one dimensional or two dimensional. A onedimensional zone is typically a stripe running horizontally across thebacklight.

The light guide 110 has one major light emitting surface, however, insome embodiments the light guide may emit light from both majorsurfaces, for example, to provide light for a double sided display. Aportion of the emissive surface of the light guide 110 can be coveredwith a reflective material. Part of the reflected light is sent(recycled) back into the light guide 110 and emits through the portionof the light guide 110 that is not covered with the reflective material,thus increasing the brightness of that portion. Also, one or more lightguide edges may have a reflector proximate or attached to the edge ofthe light guide 110 to return light transmitted out of an edge back intothe light guide 110. The light guide 110 may have one or more bevelededges to facilitate nesting, which is useful, for example, in the tiledsystems previously mentioned.

Composition of the light guide may comprise material such as acrylic,polycarbonate, cycloolefin polymer, or copolymers thereof. The lightguide may comprise an adhesive light guide. Light extraction features199 can be disposed on one or both major surfaces of the light guide110. Light extraction occurs when light is reflected by a lightextraction feature 199 at an angle that is less than the critical anglefor TIR, thus allowing the light to escape from a light guide surface.The light extraction features 199 may comprise structured features,painted features, printed features, etched features, and/or laser-madefeatures, for example. Selective absorbers can be incorporated in or onany element of the light guide 110, such as UV absorbers, anti-stat,and/or oxygen scavengers.

The light source 192 depicted in FIG. 1G may be used in conjunction withany of the optical constructions depicted herein. For simplicity, onlyone light source is illustrated in FIG. 1G, although multiple lightsources may be used. For example, only one light source that deliverslight to only one light guide edge may be used, multiple light sourcesthat deliver light to only one light guide edge may be used, only onelight source that delivers light to each of multiple light guide edgesmay be used, or multiple light sources that deliver light to multiplelight guide edges may be used. For example, a rectangular light guidehas two major surfaces and four edges. Any one or all of the four edgesmay be used as an input edge.

Light may be injected into the light guide from regions of the lightguide other than a light guide edge. FIG. 4C illustrates a light guide510 that includes a number of channels 520. Side emitting LEDs 530 arerecessed into the channels 520 and emit light 540 into the interior ofthe light guide 510 at input regions 550.

In another implementation, one or more relief shapes on the light guidemay be used to redirect light injected from a light guide surface, asillustrated in FIG. 4D. In this example, the LEDs 630 are forwardemitting, and the top surface 611 of the light guide 620 includes reliefshapes 620. The relief shapes 620 re-direct the light injected from thebottom surface 612 to within the angles for TIR. This re-directingreflection that occurs at surface relief shapes 620 can be chosen tofunction by TIR, or alternatively, a reflective coating such as silvercan be used to coat the surface relief shapes 620.

Returning to FIG. 1G, the light source 192 may be of any suitable typeand may comprise one or more cold cathode fluorescent lamps (CCFLs)and/or one or more light emitting diodes (LEDs), including LEDs thatemit red, green, blue (RGB), (red, green, blue, cyan, yellow (RGBCY),down converted light, e.g., down converted UV, blue or violet light. Thelight source 192 may comprise one or more II-VI light emitting devices,laser diodes, including vertical cavity surface emitting lasers(VCSELs), lasers, photonic lattice structures, with or without a downconverter. The light source 192 may comprise various types lightsources, e.g., LED light sources which are coupled to the light guide110 and/or embedded within the light guide 110.

A phosphor can be incorporated in the light source 192 and/or can beincluded as a remote component near the light entry region or exitregion of the light guide 110. One or more light sensors may be usedalong with a control system to collectively or independently control thelight emitted, e.g., the intensity of light emitted, by the one or morelight sources 192.

A reflector 109 may be disposed along a surface and/or one or more edgesof the light guide 110. Where multiple light sources are used, thereflector 109 may be disposed between the multiple light sources, suchas between multiple light sources disposed near an input edge. Thereflector may be specular, semi-specular or diffuse. In some cases, thelight guide 110 is attached to the reflector 109. If so, an enhancedspecular reflector (ESR) can be used. To control leaky mirror losses,the ESR can be attached to the light guide 110 using a low index layerbetween the light guide 110 and the reflector 109. In some cases, thereflector/light guide construction can be integrated into the chassis ofa notebook computer or other device which provides good structuralstability and prevents thermal warp of the optical construction and/orother display components.

The optical construction 100G includes a light redirecting film 170having structured features 171 which in this example are depicted aslinear prisms with peaks oriented away from the light guide 110 (prismsup). The structured features may be any TIR promoting replicated surfacestructures including prisms and/or lenticulars. These surface structurescan be continuous, piecewise continuous, and the dimensions of thefeatures may have chaos variation. Though primarily linear structuresare used, in-plane serpentine variations and/or variations in heightalong the peaks 172 or from peak to peak of the linear structures may beimposed. The structured features 171 are oriented so that they reflectlight rays that exit the light guide at angles less than the criticalangle required to escape the light guide, but greater than the criticalangle for TIR at the structured surface of the light redirecting layer170. These light rays do not exit from the free surface of the lightredirecting film 170, but are subject to the second tier of reflectionin the optical construction 100G. In the case of linear prisms, theorientation of the structured features that provide the second tier ofTIR is substantially parallel to the direction of light propagation 175within the light guide 110.

Optical films with prisms oriented away from the major exit surface 112of the light guide 110 can operate as light recycling films to enhancelight source brightness within a desired range of light exit angles. Theamplitude of the peaks 172 of the linear prisms 171 may vary from prismto prism, or may vary along the peak 172 of a particular prism 171. Insome examples, a first group of peaks 172 may have a height that isgreater than a second group of peaks 172. These variations may bedesirable to reduce visual defects such as wet-out or Moire effects.

The light redirecting layer 170 may be manufactured from suitablepolymeric, acrylic, polycarbonate, UV-cured acrylate, or like materials,for example. A bulk diffusing material could be incorporated into thelight redirecting layer 170, although in many cases this will degradethe performance of the optical film. Unitary, extruded layers ofacrylics and polycarbonates may be used. Alternatively, the lightredirecting layer may be a two part construction, in which thestructured surface according is cast and cured on a substrate. Forexample, ultraviolet-cured acrylics cast on polyester substrates may beused. Polyethylene terphthalate (“PET”) may be used as a substrate onwhich the structured features are cured. Biaxially oriented PET is oftenpreferred for its mechanical and optical properties. A smooth polyesterfilm that may be used as a substrate is commercially available from ICIAmericas Inc. Hopewell, Va. under the tradename MELINEX 617. A mattefinish coating that may be applied on a film to be used as a substrateis commercially available from Tekra Corporation of New Berlin, Wis.under the tradename MARNOT. 75 GU. The use of a matte finish coating mayeffect the brightness enhancement achievable using the techniquesdescribed herein, however, the matte finish may be otherwise desirablefor certain applications. The index of refraction of the lightredirecting layer 170 may be adjusted by varying the composition of thelayer.

As previously discussed, one or more intervening layers (including oneor more light redirecting layers) may be disposed between the lightredirecting film 170 and low index optical film 120.

FIG. 1H illustrates an optical construction 1 OOH, wherein the lightredirecting film 180 includes structured features 181 that are orientedtoward the major light exit surface 112 of the light guide 110 (prismsdown). Substantial portions of neighboring major surfaces of respectiveneighboring layers of optical construction 100H are in physical contactwith each other. An optical film with prisms oriented toward the majorexit surface 112 of the light guide 110 can operate as a turning filmwhich collimates light that exits from the light guide 110. Thestructured features, e.g., linear prisms 181, are arranged substantiallyperpendicular to the light propagation axis 175. In the opticalconstruction 100H, the material of the optical film 120 is disposedbetween the prisms of the turning film 180 and has a planar surface 182which is either disposed directly on the major exit surface 112 of thelight guide 110, or is disposed indirectly on the major surface 112 ofthe light guide 110 through one or more intervening layers (not shown inFIG. 1H).

FIG. 1I illustrates an optical construction 100I in which a second lowindex film 150 is disposed on a surface of the light redirecting film130. Optical construction 100I includes a light guide 110, first lowindex optical film 120, and a light redirecting film 130. One or moreadditional layers may be disposed between the layers of opticalconstruction 100I, e.g., a high absorption layer may be disposed betweenthe first low index optical film 120 and the light redirecting film 130and/or one or more adhesive layers may be disposed between the lightguide 110 and the first low index optical film 120 and/or between thefirst low index optical film 120 and the light redirecting film 130. Thesecond low index optical film 150 is disposed between the structuredfeatures 171 of the light redirecting film and planarizes the surface ofthe optical construction 100I. The optical construction 100I has theadvantage of reducing dust sensitivity and reducing high angle light.Additionally, the low index film 150 allows the construction to beattached directly to an LCD panel, such as for a cell phone. Thisconstruction may be used to provide a compact, thin, self-containedillumination/image generation module.

In some embodiments, e.g., dual-sided displays or signs, the light guidemay emit light from both major surfaces as illustrated in FIG. 1J.Optical construction 100J includes a light guide 198 with two majoremissive surfaces 197, 196. Low index films 120A, 120B are respectivelydisposed directly or indirectly on the two major emissive surfaces 197,196 of the light guide 198. Extraction features for the dual-sided lightguide may include hazy low index coating on both surfaces 197, 198, forexample. A high absorption layer and/or light redirecting films 130A,130B are disposed directly or indirectly on the low index films 120A,120B. Substantial portions of neighboring major surfaces of respectiveneighboring layers of optical construction 100J are in physical contactwith each other. Optical construction 100J is useful to provide dualsided illumination, e.g., for dual sided signs or displays. Opticaladhesive layers are optionally used between one or more of the layers ofoptical construction 100J illustrated in FIG. 1J to adhere neighboringlayers together.

FIG. 2 illustrates two-tiered reflection of light that occurs in anoptical construction in accordance with embodiments of the invention.Light is generated by light source 290 and is coupled into the lightguide 210 at a light input edge 212. The light guide 210 has index ofrefraction N1 and absorption A1.

The optical construction 200 includes low index optical film 220, whichhas an index of refraction of N2 and absorption A2 disposed directly orindirectly on the light guide 210, where N2<N1. For example, in someimplementations, the light guide 210 is acrylic, which has an index ofrefraction of 1.49. The low index optical film 220 may have an index ofrefraction less than 1.49, for example, less than 1.35, or in a range ofabout 1.10 to about 1.35. A reflective polarizer 240 is disposeddirectly or indirectly on low index optical film 220. The reflectivepolarizer 240 has an absorption A3 which is greater than the absorptionA1 of the light guide 210 and the absorption A2 of the low index opticalfilm 220. A light redirecting film 230 is disposed directly orindirectly on the reflective polarizer 240. Substantial portions ofneighboring major surfaces neighboring layers in optical construction200 are in physical contact with each other.

Optical construction 200 provides two tiers of light reflection toreduce light leaked from the light source, thereby enhancing efficiencyand reducing optical defects. The first tier of reflection occurs at themajor light exit surface 213 of light guide 210. The index ofrefraction, N2, of the low index optical film 220 is less than the indexof refraction N1 of the light guide 210. Due to the difference betweenthe refractive indices N1, N2, most of the light entering the lightguide 210 is propagated within the light guide 210 by TIR that occurs atthe light guide surface 213. However, light coupled into the light guide210 may propagate to the surface 213 at an angle of incidence that isless than the critical angle for TIR. This light is not reflected at thesurface 213 and leaks out of the light guide prematurely, i.e., beforeit is extracted by light extraction features 211. If allowed to escapefrom the optical construction 200, the prematurely extracted light wouldproduce visual artifacts and non-uniformity. The severity of theartifacts and non-uniformity would depend on the difference between therefractive index of the light guide 210 and the refraction index of thelow index optical film 220. However, this potential problem is mitigatedby the second tier of TIR which is provided by light redirecting film230.

The first tier of TIR is illustrated by the path of light ray 281 thatenters the light guide at input edge 212. Because the initial angle ofincidence 00 of light ray 281 at the major light exit surface 213 of thelight guide 210 is greater than the critical angle for TIR, the lightray 281 is reflected at the surface 213 in accordance with Snell's law.After reflection, light ray 281 continues propagating through the lightguide 210 until it strikes an extraction feature 211. Light ray 281 isreflected by extraction feature 211 and again encounters the major lightexit surface 213. However, after being reflected by the extractionfeature 212, the angle of incidence of light ray 281 at the surface 213is less than the critical angle, allowing the light ray 281 to escapefrom light guide 210. Light ray 281 may be refracted by the lightredirecting film 230 as the light ray 281 exits the optical construction200.

The second tier of TIR is illustrated by the path of light ray 282. Theinitial angle of incidence of light ray 282 at the surface 213 of thelight guide 210 is less than the critical angle for TIR allowing lightray 282 to leak from the light guide 210. Light ray 282 continues itsjourney through low index film 220 and polarizer 240 and enters lightredirecting film 230. Structured features of the light redirecting film230 are oriented to reflect light leaking from the light guide at anglesless than the critical angle at the structured surface of the lightredirecting layer 130. The angle of incidence of light ray 282 on therefractive structures of light redirecting layer is less than thecritical angle for TIR at the surface of the light redirecting film 230,and light ray 282 is reflected by the light redirecting film 230,reentering the light guide 210 and continuing its propagation until itencounters an extraction structure 211 and exits the light guide 210 atan angle that is not subject to TIR by the light redirecting film 230.

Although two tiers of light reflection are illustrated in FIG. 2, itwill be appreciated that one or more additional tiers of lightreflection may be achieved through the addition of one or moreadditional layers.

A more detailed view of a low index film is illustrated in FIG. 3. Lowindex film 300A has a porous interior by virtue of the presence ofnetwork of voids 320 within the low index film 300A. Low index film 300Amay also include a plurality of particles 340 dispersed within a binder310. In general, the low index film 300A can include one or morenetworks of interconnected pores or voids. For example, network of voids320 can be regarded to include interconnected voids or pores 320A-320C.The voids 320 may be connected to one another via hollow tunnels orhollow tunnel-like passages. The voids 320 are not necessarily free ofall matter and/or particulates. For example, in some cases, a void mayinclude one or more small fiber- or string-like objects that include,for example, a binder and/or nano-particles. In some configurations, lowindex film 300A includes multiple pluralities of interconnected voids ormultiple networks of voids where the voids in each plurality or networkare interconnected. In some cases, in addition to multiple pluralitiesof interconnected voids, the low index film 300A includes a plurality ofclosed or unconnected voids meaning that the voids are not connected toother voids via tunnels.

Low index optical film 300A supports total internal reflection (TIR) byvirtue of including a plurality of voids. When light that travels in anoptically clear non-porous medium is incident on a stratum possessinghigh porosity, the reflectivity of the incident light is much higher atoblique angles than at normal incidence. In the case of no or low hazevoided films, the reflectivity at oblique angles greater than thecritical angle is close to about 100%. In such cases, the incident lightundergoes total internal reflection (TIR).

The voids in the low index optical films have an index of refractionn_(v) and a permittivity ∈_(v), where n_(v) ²=∈v, and the binder has anindex of refraction ∈_(b) and a permittivity ∈_(b), where n_(b) ²=∈_(b).In general, the interaction of an optical film with light, such as lightthat is incident on, or propagates in, the optical film, depends on anumber of film characteristics such as, for example, the film thickness,the binder index, the void or pore index, the pore shape and size, thespatial distribution of the pores, and the wavelength of light. In somecases, light that is incident on or propagates within the optical film,“sees” or “experiences” an effective permittivity ∈_(eff) and aneffective index n_(eff), where n_(eff) can be expressed in terms of thevoid index n_(v), the binder index nb, and the film porosity or voidvolume fraction “f”. In such cases, the optical film is sufficientlythick and the voids are sufficiently small so that light cannot resolvethe shape and features of a single or isolated void. In such cases, thesize of at least a majority of the voids, such as at least 60% or 70% or80% or 90% of the voids, is not greater than about λ/5, or not greaterthan about λ/6, or not greater than about λ/8, or not greater than aboutλ/10, or not greater than about λ/20, where λ is the wavelength oflight.

In some cases, light that is incident on an optical film is a visiblelight meaning that the wavelength of the light is in the visible rangeof the electromagnetic spectrum. In such cases, the visible light has awavelength that is in a range from about 380 nm to about 750 nm, or fromabout 400 nm to about 700 nm, or from about 420 nm to about 680 nm. Insuch cases, the optical film can reasonably be assigned an effectiveindex of refraction if the size of at least a majority of the voids,such as at least 60% or 70% or 80% or 90% of the voids, is not greaterthan about 70 nm, or not greater than about 60 nm, or not greater thanabout 50 nm, or not greater than about 40 nm, or not greater than about30 nm, or not greater than about 20 nm, or not greater than about 10 nm.

In some cases, the low index optical film is sufficiently thick so thatthe optical film can reasonably have an effective index that can beexpressed in terms of the indices of refraction of the voids and thebinder, and the void or pore volume fraction or porosity. In such cases,the thickness of the low index optical film is not less than about 100nm, or not less than about 200 nm, or not less than about 500 nm, or notless than about 700 nm, or not less than about 1000 nm.

When the voids in the low index optical film are sufficiently small andthe optical film is sufficiently thick, the optical film has aneffective permittivity ∈_(eff) that can be expressed as:

∈_(eff) =f∈v+(1−f)∈b  (1)

In such cases, the effective index n_(eff) of the optical film can beexpressed as:

n _(eff) ² =fn _(v) ²+(1−f)n _(b) ²  (2)

In some cases, such as when the difference between the indices ofrefraction of the pores and the binder is sufficiently small, theeffective index of the optical film can be approximated by the followingexpression:

n _(eff) =fn _(v)+(1−f)n _(b)  (3)

In such cases, the effective index of the low index optical film is thevolume weighted average of the indices of refraction of the voids andthe binder. For example, an optical film that has a void volume fractionof about 50% and a binder that has an index of refraction of about 1.5,has an effective index of about 1.25.

In some cases, the optical haze of low index optical film 300A is notgreater than about 5%, or not greater than about 4%, or not greater thanabout 3.5%, or not greater than about 4%, or not greater than about 3%,or not greater than about 2.5%, or not greater than about 2%, or notgreater than about 1.5%, or not greater than about 1%. In such cases,the effective index of the low index optical film is not greater thanabout 1.35, or not greater than about 1.3, or not greater than about1.25, or not greater than about 1.2, or not greater than about 1.15, ornot greater than about 1.1, or not greater than about 1.05. In suchcases, the thickness of low index optical film 300A is not less thanabout 100 nm, or not less than about 200 nm, or not less than about 500nm, or not less than about 700 nm, or not less than about 1,000 nm, ornot less than about 1500 nm, or not less than about 2000 nm.

A local volume fraction of interconnected voids, for example a firstlocal volume fraction of interconnected voids 370A and a second volumefraction of interconnected voids 375A, can vary along a thickness t₁direction within low index optical film 300A. The local volume fractionof interconnected voids, and void size distribution, can vary along thethickness direction in several ways as shown, for example, in FIGS.3B-3G, described elsewhere. In some cases, the gradient optical film isa porous film meaning that the network of voids 320 forms one or morepassages between first and second major surfaces 330 and 332,respectively.

The network of voids 320 can be regarded to include a plurality ofinterconnected voids. Some of the voids can be at a surface of theoptical film 300A and can be regarded to be surface voids. For example,in the exemplary optical film 300A, voids 320D and 320E are at a secondmajor surface 332 of the low index optical film and can be regarded assurface voids 320D and 320E, and voids 320F and 320G are at a firstmajor surface 330 of the optical film 300A and can be regarded assurface voids 320F and 320G. Some of the voids, such as for examplevoids 320B and 320C, are within the interior of the optical film andaway from the exterior surfaces of the optical film and can be regardedas interior voids 320B and 320C, even though an interior void can beconnected to a major surface via, for example, other voids.

Voids 320 have a size d₁ that can generally be controlled by choosingsuitable composition and fabrication techniques, such as coating, dryingand curing conditions. In general, d₁ can be any desired value in anydesired range of values. For example, in some cases, at least a majorityof the voids, such as at least 60% or 70% or 80% or 90% or 95% of thevoids, have a size that is in a desired range. For example, in somecases, at least a majority of the voids, such as at least 60% or 70% or80% or 90% or 95% of the voids, have a size that is not greater thanabout 10 microns, or not greater than about 7 microns, or not greaterthan about 5 microns, or not greater than about 4 microns, or notgreater than about 3 microns, or not greater than about 2 microns, ornot greater than about 1 micron, or not greater than about 0.7 microns,or not greater than about 0.5 microns.

In some cases, plurality of interconnected voids 320 has an average voidor pore size that is not greater than about 5 microns, or not greaterthan about 4 microns, or not greater than about 3 microns, or notgreater than about 2 microns, or not greater than about 1 micron, or notgreater than about 0.7 microns, or not greater than about 0.5 microns.

In some cases, some of the voids can be sufficiently small so that theirprimary optical effect is to reduce the effective index, while someother voids can reduce the effective index and scatter light, whilestill some other voids can be sufficiently large so that their primaryoptical effect is to scatter light.

Particles 340 have a size d₂ that can be any desired value in anydesired range of values. For example, in some cases at least a majorityof the particles, such as at least 60% or 70% or 80% or 90% or 95% ofthe particles, have a size that is in a desired range. For example, insome cases, at least a majority of the particles, such as at least 60%or 70% or 80% or 90% or 95% of the particles, have a size that is notgreater than about 5 microns, or not greater than about 3 microns, ornot greater than about 2 microns, or not greater than about 1 micron, ornot greater than about 700 nm, or not greater than about 500 nm, or notgreater than about 200 nm, or not greater than about 100 nm, or notgreater than about 50 nm.

In some cases, plurality of particles 340 has an average particle sizethat is not greater than about 5 microns, or not greater than about 3microns, or not greater than about 2 microns, or not greater than about1 micron, or not greater than about 700 nm, or not greater than about500 nm, or not greater than about 200 nm, or not greater than about 100nm, or not greater than about 50 nm.

In some cases, some of the particles can be sufficiently small so thatthey primary affect the effective index, while some other particles canaffect the effective index and scatter light, while still some otherparticles can be sufficiently large so that their primary optical effectis to scatter light.

In some cases, d₁ and/or d₂ are sufficiently small so that the primaryoptical effect of the voids and the particles is to affect the effectiveindex of low index optical film 300A. For example, in such cases, d₁and/or d₂ are not greater than about λ/5, or not greater than about λ/6,or not greater than about λ/8, or not greater than about λ/10, or notgreater than about λ/20, where λ is the wavelength of light. As anotherexample, in such cases, d₁ and d₂ are not greater than about 70 nm, ornot greater than about 60 nm, or not greater than about 50 nm, or notgreater than about 40 nm, or not greater than about 30 nm, or notgreater than about 20 nm, or not greater than about 10 nm. In suchcases, the voids and the particles may also scatter light, but theprimary optical effect of the voids and the particles is to define aneffective medium in the optical film that has an effective index. Theeffective index depends, in part, on the indices of refraction of thevoids, the binder, and the particles. In some cases, the effective indexis a reduced effective index, meaning that the effective index is lessthan the index of the binder and the index of the particles.

In cases where the primary optical effect of the voids and/or theparticles is to affect the index, d₁ and d₂ are sufficiently small sothat a substantial fraction, such as at least about 60%, or at leastabout 70%, or at least about 80%, or at least about 90%, or at leastabout 95% of voids 320 and particles 340 have the primary optical effectof reducing the effective index. In such cases, a substantial fraction,such as at least about 60%, or at least about 70%, or at least about80%, or at least about 90%, or at least about 95% the voids and/or theparticles, have a size that is in a range from about 1 nm to about 200nm, or from about 1 nm to about 150 nm, or from about 1 nm to about 100nm, or from about 1 nm to about 50 nm, or from about 1 nm to about 20nm.

In some cases, the index of refraction n₁ of particles 340 can besufficiently close to the index n_(b) of binder 310, so that theeffective index does not depend, or depends very little, on the index ofrefraction of the particles. In such cases, the difference between n₁and n_(b) is not greater than about 0.01, or not greater than about0.007, or not greater than about 0.005, or not greater than about 0.003,or not greater than about 0.002, or not greater than about 0.001. Insome cases, particles 340 are sufficiently small and their index issufficiently close to the index of the binder, that the particles do notprimarily scatter light or affect the index. In such cases, the primaryeffect of the particles can, for example, be to enhance the strength oflow index film 300A. In some cases, particles 340 can enhance theprocess of making the low index optical film, although low index opticalfilm 300A can be made with no particles.

In cases where the primary optical effect of network of voids 320 andparticles 340 is to affect the effective index and not to, for example,scatter light, the optical haze of low index film 300A that is due tovoids 320 and particles 340 is not greater than about 5%, or not greaterthan about 4%, or not greater than about 3.5%, or not greater than about4%, or not greater than about 3%, or not greater than about 2.5%, or notgreater than about 2%, or not greater than about 1.5%, or not greaterthan about 1%. In such cases, the effective index of the effectivemedium of the low index optical film 300A is not greater than about1.35, or not greater than about 1.3, or not greater than about 1.25, ornot greater than about 1.2, or not greater than about 1.15, or notgreater than about 1.1, or not greater than about 1.05.

The thickness of the low index optical film may be not less than about100 nm, or not less than about 200 nm, or not less than about 500 nm, ornot less than about 700 nm, or not less than about 1,000 nm, or not lessthan about 1500 nm, or not less than about 2000 nm.

For light normally incident on low index optical film 300A, opticalhaze, as used herein, is defined as the ratio of the transmitted lightthat deviates from the normal direction by more than 4 degrees to thetotal transmitted light. Haze values disclosed herein were measuredusing a Haze-guard Plus haze meter (BYK-Gardiner, Silver Springs, Md.)according to the procedure described in ASTM D1003.

Low index film 300A is sufficiently thick so that the evanescent tail ofa light ray that undergoes total internal reflection at a surface of thelow index film 300A, does not optically couple, or optically couplesvery little, across the thickness of the low index film. In such cases,the thickness t1 of low index film 300A is not less than about 1 micron,or not less than about 1.1 micron, or not less than about 1.2 microns,or not less than about 1.3 microns, or not less than about 1.4 microns,or not less than about 1.5 microns, or not less than about 1.7 microns,or not less than about 2 microns. A sufficiently thick low index film300A can prevent or reduce an undesired optical coupling of theevanescent tail of an optical mode across the thickness of the low indexoptical film.

In general, low index film 300A can have any porosity or void volumefraction that may be desirable in an application. In some cases, thevolume fraction of plurality of voids 320 in low index optical film 300Ais not less than about 20%, or not less than about 30%, or not less thanabout 40%, or not less than about 50%, or not less than about 60%, ornot less than about 70%, or not less than about 80%, or not less thanabout 90%.

In some cases, low index film 300A includes a plurality of particles 340dispersed in binder 310. Particles 340 can have any size that may bedesirable in an application. For example, in some cases at least amajority of the particles, such as at least 60% or 70% or 80% or 90% or95% of the particles, have a size that is in a desired range. Forexample, in some cases, at least a majority of the particles, such as atleast 60% or 70% or 80% or 90% or 95% of the particles, have a size thatis not greater than about 5 microns, or not greater than about 3microns, or not greater than about 2 microns, or not greater than about1 micron, or not greater than about 700 nm, or not greater than about500 nm, or not greater than about 200 nm, or not greater than about 100nm, or not greater than about 50 nm.

In some cases, plurality of particles 340 has an average particle sizethat is not greater than about 5 microns, or not greater than about 3microns, or not greater than about 2 microns, or not greater than about1 micron, or not greater than about 700 nm, or not greater than about500 nm, or not greater than about 200 nm, or not greater than about 100nm, or not greater than about 50 nm.

In some cases, particles 340 are sufficiently small so that the primaryoptical effect of the particles is to affect the effective index of lowindex film 300A. For example, in such cases, particles have an averagesize that is not greater than about λ/5, or not greater than about λ/6,or not greater than about λ/8, or not greater than about λ/10, or notgreater than about λ/20, where λ is the wavelength of light. As anotherexample, the average particle size is not greater than about 70 nm, ornot greater than about 60 nm, or not greater than about 50 nm, or notgreater than about 40 nm, or not greater than about 30 nm, or notgreater than about 20 nm, or not greater than about 10 nm.

In the exemplary low index film 300A, particles 340, such as particles340A and 340B, are solid particles. In some cases, low index opticalfilm 300A may additionally or alternatively include a plurality ofhollow or porous particles 350.

Particles 340 can be any type particles that may be desirable in anapplication. For example, particles 340 can be organic or inorganicparticles. For example, particles 340 can be silica, zirconium oxide oralumina particles.

Particles 340 can have any shape that may be desirable or available inan application. For example, particles 340 can have a regular orirregular shape. For example, particles 340 can be approximatelyspherical. As another example, the particles 340 can be elongated. Insuch cases, low index optical film 300A includes a plurality ofelongated particles 320. In some cases, the elongated particles have anaverage aspect ratio that is not less than about 1.5, or not less thanabout 2, or not less than about 2.5, or not less than about 3, or notless than about 3.5, or not less than about 4, or not less than about4.5, or not less than about 5. In some cases, the particles 340 can bein the form or shape of a string-of-pearls (such as Snowtex-PS particlesavailable from Nissan Chemical, Houston, Tex.) or aggregated chains ofspherical or amorphous particles, such as fumed silica.

Particles 340 may or may not be functionalized. In some cases, particles340 are not functionalized. In some cases, particles 340 arefunctionalized so that they can be dispersed in a desired solvent orbinder 310 with no, or very little, clumping. In some cases, particles340 can be further functionalized to chemically bond to binder 310. Forexample, particles 340, such as particle 340A, can be surface modifiedand have reactive functionalities or groups 360 to chemically bond tobinder 310. In such cases, at least a significant fraction of particles340 is chemically bound to the binder. In some cases, particles 340 donot have reactive functionalities to chemically bond to binder 310. Insuch cases, particles 340 can be physically bound to binder 310, orbinder 310 can encapsulate particles 340.

In some cases, some of the particles 340 have reactive groups and othersdo not have reactive groups. For example in some cases, about 10% of theparticles have reactive groups and about 90% of the particles do nothave reactive groups, or about 15% of the particles have reactive groupsand about 85% of the particles do not have reactive groups, or about 20%of the particles have reactive groups and about 80% of the particles donot have reactive groups, or about 25% of the particles have reactivegroups and about 75% of the particles do not have reactive groups, orabout 30% of the particles have reactive groups and about 60% of theparticles do not have reactive groups, or about 35% of the particleshave reactive groups and about 65% of the particles do not have reactivegroups, or about 40% of the particles have reactive groups and about 60%of the particles do not have reactive groups, or about 45% of theparticles have reactive groups and about 55% of the particles do nothave reactive groups, or about 50% of the particles have reactive groupsand about 50% of the particles do not have reactive groups, or about 55%of the particles have reactive groups and about 45% of the particles donot have reactive groups, or about 60% of the particles have reactivegroups and about 40% of the particles do not have reactive groups, orabout 65% of the particles have reactive groups and about 35% of theparticles do not have reactive groups, or about 70% of the particleshave reactive groups and about 30% of the particles do not have reactivegroups, or about 75% of the particles have reactive groups and about 25%of the particles do not have reactive groups, or about 80% of theparticles have reactive groups and about 20% of the particles do nothave reactive groups, or about 85% of the particles have reactive groupsand about 15% of the particles do not have reactive groups, or about 90%of the particles have reactive groups and about 10% of the particles donot have reactive groups.

In some cases, some of the particles may be functionalized with bothreactive and unreactive groups on the same particle.

The ensemble of particles may include a mixture of sizes, reactive andnon-reactive particles and different types of particles, for example,organic particles including polymeric particles such as acrylics,polycarbonates, polystyrenes, silicones and the like; or inorganicparticles such as glasses or ceramics including, for example, silica andzirconium oxide, and the like.

Binder 310 can be or include any material that may be desirable in anapplication. For example, binder 310 can be a curable material thatforms a polymer, such as a cross-linked polymer. In general, binder 310can be any polymerizable material, such as a polymerizable material thatis radiation-curable, such as a UV curable material.

In general, the weight ratio of binder 310 to plurality of particles 340can be any ratio that may be desirable in an application. In some cases,the weight ratio of the binder to the plurality of the particles is notless than about 1:1, or not less than about 1.5:1, or not less thanabout 2:1, or not less than about 2.5:1, or not less than about 3:1, ornot less than about 3.5:1, or not less than about 4:1.

In some cases, low index optical film 300A includes a binder, a fumedmetal oxide such as a fumed silica or alumina, and a plurality ornetwork of interconnected voids. In such the weight ratio of the fumedmetal oxide to the binder is in a range from about 2:1 to about 6:1, orin a range from about 2:1 to about 4:1. In some cases, the weight ratioof the fumed metal oxide to the binder is not less than about 2:1, ornot less than about 3:1. In some cases, the weight ratio of the fumedmetal oxide to the binder is not greater than about 8:1, or not greaterthan about 7:1, or not greater than about 6:1.

In some cases, low index optical film 300A can be or include a porouspolypropylene and/or polyethylene film such as a CELGARD film availablefrom Celanese Separation Products of Charlotte, N.C.). For example, lowindex optical film 300A can be or include a CELGARD 2500 film having athickness of about 25 microns and 55% porosity. As another example, lowindex optical film 300A can be or include a CELGARD M824 film having athickness of about 12 microns and 38% porosity. FIG. 5 is an exemplaryoptical image of a CELGARD film.

In some cases, low index optical film 300A can be or include a porousfilm that is made by thermally induced phase separation (TIPS), such asthose made according to the teachings of U.S. Pat. Nos. 4,539,256 and5,120,594. TIPS films can have a broad range of microscopic pore sizes.FIG. 6 is an exemplary optical image of a TIPS film.

In some cases, low index optical film 300A can be or include a porousfilm that is made by solvent induced phase separation (SIPS), anexemplary optical micrograph of which is shown in FIG. 7. In some cases,optical film 120 can be or include a polyvinylidene fluoride (PVDF)porous film.

Low index optical film 300A can comprise the optical films described inco-pending applications: “Optical Film” (Attorney Docket No. 65062US002)filed Apr. 15, 2009 and having Ser. No. 61/169,466; “Gradient Low IndexArticle and Method” (Attorney Docket No. 65716US002) filed Oct. 24, 2009and having Ser. No. 61/254,673; U.S. Provisional Application No.61/169,532 entitled “Retroreflecting Optical Construction” (AttorneyDocket No. 65355US002) filed Apr. 15, 2009 which are incorporated hereinby reference.

Low index optical film 300A can be produced using any method that may bedesirable in an application. In some cases, optical film 300A can beproduced by the processes described in co-pending applications: U.S.Provisional Application No. 61/169,429 entitled “PROCESS AND APPARATUSFOR A NANOVOIDED ARTICLE”, Attorney Docket Number 65046US002, filed Apr.15, 2009; U.S. Provisional Application No. 61/169,427 entitled “PROCESSAND APPARATUS FOR COATING WITH REDUCED DEFECTS”, Attorney Docket Number65185US002, filed Apr. 15, 2009; and U.S. Provisional Application No.61/254,674 entitled “PROCESS FOR GRADIENT NANOVOIDED ARTICLE”, AttorneyDocket No. 65766US002 filed Oct. 24, 2009, the disclosures of which areincorporated in their entirety herein by reference.

For example, the low index optical film may comprise a gel-type film asdescribed in U.S. Provisional Application No. 61/169,466 entitled“Optical Film” (Attorney Docket No. 65062US002 and produced by processesdescribed in U.S. Provisional Application No. 61/169,429 entitled“PROCESS AND APPARATUS FOR A NANOVOIDED ARTICLE”, Attorney Docket Number65046US002). The low index optical film may comprise a fumed silica filmdescribed in U.S. Provisional Application No. 61/169,532 entitled“Retroreflecting Optical Construction” (Attorney Docket No. 65355US002).

Generally, in one process, first a solution is prepared that includes aplurality of particles, such as nano-particles, and a polymerizablematerial dissolved in a solvent, where the polymerizable material caninclude, for example, one or more types of monomers. Next, thepolymerizable material is polymerized, for example by applying heat orlight, to form an insoluble polymer matrix in the solvent. In oneparticular embodiment, the polymerization occurs in an environment thathas an elevated level of oxygen adjacent one of the surfaces, inhibitingthe polymerization near that surface to create a gradient optical film.In one particular embodiment, a concentration of photoinitiator near oneof the surfaces is increased relative to another surface, to create agradient optical film.

In some cases, after the polymerization step, the solvent may stillinclude some of the polymerizable material, although at a lowerconcentration. Next, the solvent is removed by drying or evaporating thesolution resulting in low index optical film 300A that includes anetwork, or a plurality, of voids 320 dispersed in polymer binder 310.The gradient optical film further includes plurality of particles 340dispersed in the polymer. The particles are bound to the binder, wherethe bonding can be physical or chemical, or be encapsulated by thebinder.

Low index optical film 300A can have other materials in addition tobinder 310 and particles 340. For example, low index optical film 300Acan include one or more additives, such as for example, coupling agents,to help wet the surface of a substrate on which the gradient opticalfilm is formed. As another example, low index optical film 300A caninclude one or more colorants, such a carbon black, for imparting acolor, such as the black color, to the low index optical film 300A.Other exemplary materials in low index optical film 300A includeinitiators, such as one or more photo-initiators, anti-stats, UVabsorbers and release agents. In some cases, low index optical film 300Acan include a down converting material that is capable of absorbinglight and reemitting a longer wavelength light. Exemplary downconverting materials include phosphors.

In general, low index optical film 300A can have a desirable porosityfor any weight ratio of binder 310 to plurality of particles 340.Accordingly, in general, the weight ratio can be any value that may bedesirable in an application. In some cases, the weight ratio of binder310 to plurality of particles 340 is not less than about 1:2.5, or notless than about 1:2.3, or not less than about 1:2, or not less thanabout 1:1, or not less than about 1.5:1, or not less than about 2:1, ornot less than about 2.5:1, or not less than about 3:1, or not less thanabout 3.5:1, or not less than about 4:1, or not less than about 5:1. Insome cases, the weight ratio is in a range from about 1:2.3 to about4:1.

In some cases, top major surface 332 of low index optical film 300A canbe treated to, for example, improve the adhesion of the low indexoptical film to another layer. For example, the top surface can becorona treated.

In some embodiments, the low index optical films have a substantiallyconstant porosity along a thickness direction. In other embodiments, thelow index optical films exhibit a local porosity that varies along athickness direction of the low index optical films. Optical filmsexhibiting a variation in local porosity along the thickness directionare denoted herein as gradient films or gradient low index films. Insome cases, the local porosity may be described by a local void volumefraction, or as a local pore size distribution. For example, a gradientoptical film may have a first local volume fraction and a second localvolume fraction, wherein the second volume fraction of the plurality ofvoids is less than 50%, 20%, 10%, or 1% of the first volume fraction.

FIGS. 3B-3G are schematic side-views of a gradient low index film300B-300G, respectively, according to different aspects of thedisclosure. For clarity, the numbered elements 310-360 and the sizesd₁-d₃ described for FIG. 3A are not shown in FIGS. 3B-3G; however, eachof the descriptions provided for low index optical film 300A of FIG. 3Aalso correspond to the gradient optical film 300B-300G of FIGS. 3B-3G,respectively. Techniques for creating the gradient optical films300B-300G are described, for example, in co-pending U.S. ProvisionalApplication No. 61/254,674 entitled “PROCESS FOR GRADIENT NANOVOIDEDARTICLE”, Attorney Docket No. 65766US002.

In FIG. 3B, gradient low index optical film 300B includes a local volumefraction of interconnected voids 390B that varies along the thicknessdirection, for example, in a monotonic manner as shown. In oneparticular embodiment, a first local volume fraction of interconnectedvoids 370B proximate a first surface 330B of gradient low index opticalfilm 300B is lower than a second local volume fraction of interconnectedvoids 375B proximate a second surface 332B of gradient low index opticalfilm 300B.

Gradient low index optical film 300B can be prepared using a variety oftechniques, as described elsewhere. In one particular embodiment,gradient low index optical film 300B can be prepared, for example, usingan absorbance based technique where the intensity of polymerizationlight decreases from first surface 330B to second surface 332B.

In FIG. 3C, gradient low index optical film 300C includes a local volumefraction of interconnected voids 390C that varies along the thicknessdirection, for example, in a step-wise manner as shown. In oneparticular embodiment, a first local volume fraction of interconnectedvoids 370C proximate a first surface 330C of gradient optical film 300Cis lower than a second local volume fraction of interconnected voids375C proximate a second surface 332C of gradient optical film 300C. Insome cases, for example, shown FIG. 1C, first local volume fraction ofinterconnected voids 370C transitions sharply (i.e., step-wise) tosecond local volume fraction of interconnected voids 375C. In somecases, a thickness t2 of the second volume fraction of interconnectedvoids 375C can be a small percentage of the total thickness t₁, forexample, from about 1% to about 5%, or to about 10%, or to about 20%, orto about 30% or more of the total thickness t₁.

Gradient low index optical film 300C can be prepared using a variety oftechniques, as described elsewhere. In one particular embodiment,gradient low index optical film 300C can be prepared, for example, byusing a difference in the polymerization initiator concentration or adifference in the polymerization inhibitor concentration proximate thefirst and second surfaces (330C, 332C).

In FIG. 3D, gradient low index optical film 300D includes a local volumefraction of interconnected voids 390D that varies along the thicknessdirection, for example, having a minimum local volume fraction ofinterconnected voids 377D as shown. In one particular embodiment, afirst local volume fraction of interconnected voids 370D proximate afirst surface 330D of gradient optical film 300D is approximately thesame as a second local volume fraction of interconnected voids 375Dproximate a second surface 332D of gradient low index optical film 300D.In some cases, for example, shown FIG. 3D, first local volume fractionof interconnected voids 370D transitions sharply (i.e., step-wise) tominimum local volume fraction of interconnected voids 377D. In somecases, a thickness t₂ of the minimum volume fraction of interconnectedvoids 377D can be a small percentage of the total thickness t₁, forexample, from about 1% to about 5%, or to about 10%, or to about 20%, orto about 30% or more of the total thickness t₁. In some cases, therelative position of the minimum local volume fraction of interconnectedvoids 377D can be located anywhere, for example, at thickness t3 fromfirst surface 330D, within gradient optical film 300D.

Gradient low index optical film 300D can be prepared using a variety oftechniques, as described elsewhere. In one particular embodiment,gradient optical film 300D can be prepared, for example, by laminating apair of the gradient optical films 300C shown in FIG. 3C to each other,along the second surfaces 332C.

In FIG. 3E, gradient low index optical film 300E includes a local volumefraction of interconnected voids 390E that varies along the thicknessdirection, for example, having a step-change local volume fraction ofinterconnected voids proximate a first and a second surface 330E, 332E,as shown. In one particular embodiment, a first local volume fraction ofinterconnected voids 370E proximate a first surface 330E of gradientoptical film 300E is approximately the same as a second local volumefraction of interconnected voids 375E proximate a second surface 332E ofgradient optical film 300E. In some cases, for example, shown FIG. 3E,first local volume fraction of interconnected voids 370E transitionssharply (i.e., step-wise) to maximum local volume fraction ofinterconnected voids 377E. In some cases, a thickness t₂ and t₃ of thefirst and second local volume fraction of interconnected voids 370E and375E, respectively, can be a small percentage of the total thickness t₁,for example, from about 1% to about 5%, or to about 10%, or to about20%, or to about 30% or more of the total thickness t1. In some cases,each of the first and second local volume fraction of interconnectedvoids 370E and 375E can have transitions that are not step-wise (notshown, but similar to the monotonic variation shown in FIG. 3B).

Gradient low index optical film 300E can be prepared using a variety oftechniques, as described elsewhere. In one particular embodiment,gradient optical film 300E can be prepared, for example, by laminating apair of the gradient optical films 300C shown in FIG. 3C to each other,along the first surfaces 330C.

In FIG. 3F, gradient low index optical film 300F includes a local volumefraction of interconnected voids 390F that varies along the thicknessdirection, for example, having a gradient minimum local volume fractionof interconnected voids 377F as shown. In one particular embodiment, afirst local volume fraction of interconnected voids 370F proximate afirst surface 330F of gradient optical film 300F is approximately thesame as a second local volume fraction of interconnected voids 375Fproximate a second surface 332F of gradient optical film 300F. In somecases, for example, shown FIG. 3F, first local volume fraction ofinterconnected voids 370F transitions gradually (i.e., in a monotonicgradient) to a minimum local volume fraction of interconnected voids377F, and again transitions gradually to the second volume fraction ofinterconnected voids 375F.

Gradient low index optical film 300F can be prepared using a variety oftechniques, as described elsewhere. In one particular embodiment,gradient optical film 300F can be prepared, for example, by laminating apair of the gradient optical films 300B shown in FIG. 3B to each other,along the second surfaces 332B.

In FIG. 3G, gradient low index optical film 300G includes a local volumefraction of interconnected voids 390G that varies along the thicknessdirection, for example, having a pair of step-change local volumefraction of interconnected voids 377G, 378G, as shown. In one particularembodiment, a first local volume fraction of interconnected voids 370Gproximate a first surface 330G of gradient optical film 300G isapproximately the same as a second local volume fraction ofinterconnected voids 375G proximate a second surface 332G of gradientoptical film 300G. In some cases, for example, shown in FIG. 3G, firstlocal volume fraction of interconnected voids 370G transitions sharply(i.e., step-wise) to minimum local volume fraction of interconnectedvoids 377G, transitions sharply again to a maximum local volume fractionof interconnected voids 380G, transitions sharply again to a minimumlocal volume fraction of interconnected voids 378G, and finallytransitions sharply yet again to the second local volume fraction ofinterconnected voids 375G. In some cases, each of the local volumefraction of interconnected voids can have transitions that are notstep-wise (not shown, but similar to the monotonic variation shown inFIG. 3B).

Gradient low index optical film 300G can be prepared using a variety oftechniques, as described elsewhere. In one particular embodiment,gradient optical film 300G can be prepared, for example, by a multilayercoating technique, where a different photoinitiator concentration can beused in strata corresponding to minimum local void volume fraction(377G, 378G) than in strata corresponding to maximum local void volumefraction 390G.

FIG. 8A is a cross-sectional micrograph of a gradient low index opticalfilm 800 coated on a substrate 810, according to one aspect of thedisclosure. The gradient optical film 800 includes a first major surface830 adjacent to the substrate 810, and a first local volume fraction ofinterconnected voids 870 proximate the first major surface 830. Thegradient optical film further includes a second major surface 832 and adensified second local volume fraction of interconnected voids 875proximate the second major surface 832. FIG. 8B is a highermagnification of the micrograph in FIG. 8A, and more clearly shows thatthe first local volume fraction of interconnected voids 870 is greaterthan the densified second volume fraction of interconnected voids 875.

Optical constructions described herein are useful in display backlights,e.g., for LCD displays. Confining light to a zone within a backlight isan important capability in some applications. Doing so enables dynamicdimming, where the backlight is actively, locally reduced in brightnessdepending on display content. This has the advantage of significantlyreducing display power, and can increase contrast. Additionally, fieldsequential systems can benefit from zoning, and with proper pixelswitching speeds can enable rendition of fast motion images with reducedimage blur.

Optical constructions as described herein can be arranged as elements inan array of 10 s to 100 s of light guides to create a zoned system. Eachelement of the array may include one or more light sources, a lightguide, and one or more optical layers, e.g., a low index optical film,polarizer, and light redirecting film. In these configurations, thebrightness of each element can be individually controlled. In somecases, uniformity is enhanced by nesting and/or tiling the light guidesso that the LEDs and part of the light guide are positioned under anadjacent light guide. This interleaved, or sawtooth pattern replicatesthroughout the display. Two scenarios are shown below in FIGS. 4A and4B. The size of the light guides used in these implementations can besmall, on the order of one to several cm diagonal, or can be hundreds ofcm for large wall displays where each element can be the size of a TV.

FIG. 4A illustrates nested optical constructions 410, 415. Each of theoptical constructions 410, 411 includes a light guide 411, 416, lightsource 490, 491, and one or more optical layers 412, 417, which mayinclude a low index film, a light redirecting film and/or a polarizer. Aliquid crystal display (LCD) panel 480 is positioned above the nestedoptical constructions 410, 415.

As illustrated in FIG. 4A, light source 491 and a portion of light guide416 are positioned under adjacent optical construction 410. Thepositioning of light source 491 and light guide 416 under an adjacentlight guide 411 involves sliding the tiles together so that little or nogap exists between the tiles. There may be a shield such as a reflectivestrip positioned directly above the light source 490, 491 to limit anylight leaked directly toward the LCD panel 480.

FIG. 4B illustrates tiled optical constructions 420, 425, 430. Each ofthe optical constructions 420, 425, 430 includes a light guide 421, 426,431, light source 492, 493, 494, and one or more optical layers 422,427, 432 which may comprise a low index film, a light redirecting filmand/or a polarizer.

Examples 1. Materials Used in Preparation of the Examples and Acronyms

Prism film on 10 mil polycarbonate (denoted herein as PC-BEF)—A curablelayer with a linear prismatic structured surface was coated and cured ona 10 mil polycarbonate substrate. The uncured material had an index ofrefractive of greater than 1.55. The linear prismatic structures had a90° included angle, 50 micron pitch, and a tip radius of 7 microns. Theprism film was made as described in U.S. Pat. No. 6,280,063 on a 10 milpolycarbonate film.

Adhesive (denoted herein in as PSA)—0.1% bisamide crosslinker was addedto SK Dyne 2003K wet adhesive, available from Soken Chemicals, Tokyo,Japan, and the mixture was coated onto 2 mil polyester silicone releaseliner (T50 available from CP Films, St. Louis, Mo.) using a conventionalslot die and the solvent was dried, leaving a 1 mil thick adhesivecoating. A second release liner was laminated to the surface of thedried adhesive: 2 mil polyester silicone release liner with differentialrelease (T10 also available from CP Films).

Reflective polarizer A (denoted herein as DBEF-Q)—DBEF-Q is a reflectivepolarizer (available as Vikuiti DBEF-Q from 3M Company St. Paul, Minn.).The DBEF-Q was about 93 microns thick.

Low index film on 2 mil PET (ULI)—In a 2 liter three-neck flask,equipped with a condenser and a thermometer, 960 grams of IPA-ST-UPorganosilica elongated particles (available from Nissan Chemical Inc.,Houston, Tex.), 19.2 grams of deionized water, and 350 grams of1-methoxy-2-propanol were mixed under rapid stirring. The elongatedparticles had a diameter in a range from about 9 nm to about 15 nm and alength in a range of about 40 nm to about 100 nm. The particles weredispersed in a 15.2% wt IPA. Next, 22.8 grams of Silquest A-174 silane(available from GE Advanced Materials, Wilton, Conn.) was added to theflask. The resulting mixture was stirred for 30 minutes. The mixture waskept at 81° C. for 16 hours. Next, the solution was allowed to cool downto room temperature. Next, about 950 grams of the solvent in thesolution were removed using a rotary evaporator under a 40° C.water-bath, resulting in a 42.1% wt A-174-modified elongated silicaclear dispersion in 1-methoxy-2-propanol. Next, 47.5 grams of this cleardispersion, 16 grams of SR 444 (available from Sartomer Company, Exton,Pa.), 4 grams of CN2261 (available from Sartomer Company, Exton, Pa.),30 grams of isopropyl alcohol, 30 grams of ethyl acetate, 0.6 grams ofphotoinitiator Irgacure 184 and 0.1 grams of photoinitiator Irgacure 819(both available from Ciba Specialty Chemicals Company, High Point N.C.)were mixed together and stirred resulting in a homogenous coatingsolution with 31.3% wt solids. Next, the coating solution was coated ona 2 mil (0.051 mm) thick PET substrate using the coating method asdescribed below. The coating solution was syringe-pumped at a rate of 6cc/min into a 20.3 cm (8-inch) wide slot-type coating die. The slotcoating die uniformly distributed a 20.3 cm wide coating onto asubstrate moving at 10 ft/min (152 cm/min). Next, the coating waspolymerized by passing the coated substrate through a UV-LED curechamber that included a quartz window to allow passage of UV radiation.The UV-LED bank included a rectangular array of 352 UV-LEDs, 16 down-webby 22 cross-web (approximately covering a 20.3 cm×20.3 cm area). TheUV-LEDs were placed on two water-cooled heat sinks. The LEDs (availablefrom Cree, Inc., Durham N.C.) operated at a nominal wavelength of 395nm, and were run at 45 Volts at 13 Amps, resulting in a UVA dose of0.1352 joules per square cm. The UV-LED array was powered and fan-cooledby a TENMA 72-6910 (42V/10 A) power supply (available from Tenma,Springboro Ohio).

The UV-LEDs were positioned above the cure chamber quartz window at adistance of approximately 2.54 cm from the substrate. The UV-LED curechamber was supplied with a flow of nitrogen at a flow rate of 46.7liters/min (100 cubic feet per hour) resulting in an oxygenconcentration of approximately 150 ppm in the cure chamber.

After being polymerized by the UV-LEDs, the solvent in the cured coatingwas removed by transporting the coating to a drying oven operating at150° F. for 2 minutes at a web speed of 10 ft/min. Next, the driedcoating was post-cured using a Fusion System Model I300P configured withan H-bulb (available from Fusion UV Systems, Gaithersburg Md.).

The UV Fusion chamber was supplied with a flow of nitrogen that resultedin an oxygen concentration of approximately 50 ppm in the chamber. Theresulting optical film had a total optical transmittance of about 94.9%,an optical haze of 1.1%, a refractive index of 1.155, and a thickness ofabout 6 microns.

Reflective polarizer B (2×TOP)—A multilayer reflective polarizer wasmade according as described in PCT Patent Application WO2009/123928.

The thickness was 29 microns. A multilayer reflective polarizer was madeaccording to WO2009/123928. The reflection band extended from 400 to1200 nm. Two ply of this film were laminated together using PSA adhesiveto result in the 2×TOP construction.

Light guide plate (LGP)—The LGP was obtained from Coretronic Company(Hsinchu, Taiwan 300, R.O.C.), model AUT1982T32. The LGP is made ofpoly(methyl methacrylate) with white print dots on the bottom surface, 6mm thick, 385 mm wide, and 306 mm long.

White back reflector: (WBR)—The WBR was obtained from Viewsonic 22 inchmonitor (model#: VLED221 wm), available from Viewsonic Company, Walnut,Calif., USA.

Backlight—A 22″ Viewsonic monitor model # VLED221 wm was disassembled toseparate the backlight from the panel. The backlight was sizedapproximately 473 mm wide by 306 mm long. It contained a row of 78 LEDs,with the pitch of ˜6 mm, along each 473 mm edge of the backlight and aWBR lining the back wall. Only 63 LEDs on each edge were used for theexamples.

Examples 1 through 4 include a multilayer reflective polarizer laminatedto the LGP, where the pass-axis of the reflective polarizer is alignedwith the 385 mm length. The polarized LGP is then set into the backlighton top of the WBR such that the pass axis of the multilayer reflectivepolarizer is parallel to the rows of LEDs.

For experimental simplicity, material availability, and ease ofinterpretation, the experimental examples 1-4 use constructions thatinclude a relatively high number of layers. Experimental constructionswere made that would ensure that the same low index film coating wasapplied in all examples, and modify the orientation of prisms whennecessary. Optical constructions could be made without the PET layer andthe extra adhesive layers required for the PET. Additionally, prismscould be cast directly on the MOF, eliminating the prism substrate. Lowindex coatings and prism can be directly coated on MOF with no need forlamination, resulting in simple and practical constructions.

In all experimental examples from 1 to 4, the MOFs were arranged suchthat their pass axis was parallel to the long side of the solid LGP,where light engines were installed.

Example 1 (Experiment)

An optical construction 900 (see FIG. 9A) having PC-BEF (prismsperpendicular to the direction of light propagation in the LGP), DBEF-Q,and clear low index film coating (designated ULI), with a separate whiteback reflector and conventional extraction features on LGP wasfabricated and tested. This example shows that even with low indexoptical coating, the concept with prisms running perpendicular to thedirection of light propagation in the LGP exhibited a hall-of-mirroreffect of multiple LED images due to the fact that certain amount oflight is not guided through total internal reflection (TIR) at thesurface of the LGP, and the orientation of the PC-BEF did not provide anadequate second tier of TIR.

A schematic side-view of optical construction 900 is shown FIG. 9A. Theoptical construction included a WBR, a LGP, LED lamps, prism film,DBEF-Q, and low-index optical coating (ULI). Sixty-three LEDs wereplaced near each edge of the LGP and arranged regularly along the widthof the LGP (385 mm).

A picture of the optical construction 900 with LEDs being powered on,was taken with a digital camera (Canon S550) taken at approximately 60degrees from normal to the surface of the optical construction. Thepicture is shown in FIG. 9B. The appearance of multiple images of LEDs(hall-of-mirror) results from prisms running perpendicular to thedirection of light propagation in the LGP.

Example 2a (Experiment)

An optical construction 1000 (FIG. 10A) using PC-BEF (prisms orientedparallel to the direction of light propagation in the LGP), DBEF-Q, andclear low index film coating (ULI), with a separate white back reflectorand conventional extraction features was fabricated and tested. The Hallof mirror (multiple images of LEDs) was substantially eliminated in thisexample.

A schematic side-view of the optical construction 1000 tested in Example2a is shown in FIG. 10A. The optical construction 1000 was similar tooptical construction 900 of Example 1 except that the prisms of opticalconstruction 1000 were running parallel to the direction of lightpropagation in the LGP.

The luminance of the optical construction as a function of viewing anglewas measured using an Autronic Conoscope Conostage 3 (available fromAutronic-Melchers GmbH, Karlsruhe, Germany). Before making themeasurements, a linear absorbing polarizer, not shown expressly in FIG.10A, was placed on top of the optical construction with its pass-axisparallel to the long side of the LGP. FIG. 10B is a grayscale image ofthe measured luminance as a function of viewing angle, FIG. 10C is agraph of the measured luminance as a function of viewing angle along thehorizontal direction. FIG. 10D is a graph of the measured luminance as afunction of viewing angle along the vertical direction. PC-BEF waschosen as the light redirecting film because it does not significantlydepolarize light.

This example demonstrated several enhancements for backlightapplications: (1) substantial reduction of hall-of-mirror effect; (2)collimation of light with prism film; (3) polarized output.

Example 2b (Experiment)

An optical construction 1100 (FIG. 11A) using PC-BEF (prisms parallel tothe direction of light propagation in the LGP), 2 X TOP, and clear lowindex film coating (ULI), with a separate white back reflector (WBR) andconventional extraction features was fabricated and tested.

Optical construction 1100 was similar to optical construction 1000 inexample 2a except that DBEF was replaced by 2×TOP. A schematic side-viewof the optical construction 1100 is shown in FIG. 11A. FIG. 11B is agrayscale image of the measured luminance as a function of viewingangle, FIG. 11C is a graph of the measured luminance as a function ofviewing angle along the horizontal direction. FIG. 11D is a graph of themeasured luminance as a function of viewing angle along the verticaldirection. Optical construction 1100 shows better collimation fromcollimating 2×TOP compared against DBEF-Q in optical construction 1000(Example 2a).

Example 3 (Experiment)

An optical construction 1200 (FIG. 12A) using PC-BEF (prismsperpendicular to the direction of light propagation in the LGP), andDBEF-Q, with a separate white back reflector and conventional extractionfeatures was fabricated and tested.

This example illustrates the optical construction 1200 with prismsrunning perpendicular to the direction of propagating in the LGPproduces a hall-of-mirror effect (multiple LED images) caused by lightthat is not guided through total internal reflection in the LGP.

Optical construction 1200, a schematic side-view of which is shown inFIG. 12A was made. Optical construction 1200 in example 3 was similar tooptical construction 900 in Example 1 except that a low index opticalcoating (ULI) on PET was not used. A picture of optical construction1200 with LEDs powered on, taken with a digital camera (Canon S550) atapproximately 60 degrees from surface normal, is shown in FIG. 12Billustrating multiple images of the LEDs (hall-of-mirror).

Example 4 (Experiment)

An optical construction 1300 (FIG. 13A) using PC-BEF (prisms parallel tothe direction of light propagation in the LGP), and DBEF-Q, with aseparate white back reflector and conventional extraction features wasfabricated and tested. The prism film failed to effectively collimatethe light with no low index coating.

Measurements similar to those described in Example 2a were made and agrayscale image of the measured luminance as a function of viewing angleis depicted in (FIG. 13B). Cross sections along horizontal and verticaldirections are shown in FIGS. 13C and 13D, respectively.

Examples 5-12 are based on simulations. Simulations 5-12 were performedusing LightTools 6.0, commercially available ray-tracing software fromOptical Research Associates, Calif., USA.

Example 5 (Modeling)

The simulated optical construction included a light redirecting layeroriented with prisms perpendicular to the direction of light propagationin the LGP, low index film (ULI) with a refractive index (RI) of 1.0,and an LGP without extractors.

Nine LEDs with Lambertian emission profile, height of 3 mm and width of3 mm, pitch of mm, were placed closely to the left edge of a solid lightguide plate (LGP) with the refractive index (RI) of 1.49 (PMMA). The LGPwas 6 mm thick (y-direction), 90 mm wide (z-direction), and 300 mm long(x-direction.) A prism film with RI of 1.567 and 90° included angle wasattached to the top surface of the LGP with low index coating inbetween. The prisms were oriented perpendicular to the direction oflight propagation in the LGP. FIG. 14 shows the construction and raytracing modeling result for Example 5. A total of 100 rays were emittedfrom the LEDs in this modeling. With the low index film RI of 1.0,corresponding to air, all light emitted by the LEDs and entering the LGPfrom the left edge is guided between the LGP bottom and LGP/low indexinterface by total internal reflection (TIR).

Example 6 (Modeling)

The simulated optical construction included a light redirecting layeroriented with prisms perpendicular to the direction of light propagationin the LGP, low index film (ULI) with a refractive index (RI) of 1.109,and LGP without extractors.

With the same construction as Example 5 except that the low index filmRI is increased to 1.109, all light emitted by the LEDs and entering theLGP from the left edge is guided between the LGP bottom and LGP/lowindex interface by TIR. According to simulations, a low index film RIabove 1.109 caused light to leak through LGP/low index film interface.FIG. 15 shows the construction and ray tracing modeling result forExample 6.

Example 7 (Modeling)

The simulated optical construction included a light redirecting layeroriented with prisms perpendicular to the direction of light propagationin the LGP, low index film (ULI) refractive index (RI) of 1.20, and anLGP w/o extractors.

With the same construction as Example 5 except that the low index filmRI is increased to 1.2, most of light emitted by the LEDs and enteringthe LGP from the left edge is guided between the LGP bottom and LGP/lowindex film interface by TIR; about 5% of light leaks through LGP/lowindex film interface and not TIR guided, as shown in FIG. 16, which isthe cause of hall-of-mirror (multiple images of LEDs). FIG. 16 shows theconstruction and ray tracing modeling result for Example 7.

Example 8 (Modeling)

The simulated optical construction included light redirecting filmoriented with prisms parallel to the direction of light propagation inthe LGP, a low index film (ULI) refractive index (RI) of 1.0, and an LGPwithout extractors.

With the same construction as Example 5 except that the lightredirecting film was oriented with prisms running parallel to thedirection of propagation of light in the LGP, all light emitted by theLEDs and entering the LGP from the left edge is guided between the LGPbottom and LGP/low index film interface by TIR. FIG. 17 shows theoptical construction and ray tracing modeling result for Example 8.

Example 9 (Modeling)

The simulated optical construction included a light redirecting filmoriented with prisms parallel to the direction of light propagation inthe light guide, a low index film (ULI) refractive index (RI) of 1.109,and an LGP w/o extractors.

With the same construction as Example 8 except that the low index filmRI is increased to 1.109, all light emitted by the LEDs and entering theLGP from the left edge is guided between the LGP bottom and LGP/lowindex film interface by TIR. Modeling indicates that a low index film RIof 1.109 is a value above which light would start leaking throughLGP/low index film interface. FIG. 18 shows the construction and raytracing modeling result for Example 9.

Example 10 (Modeling)

The simulated optical construction included a light redirecting filmoriented with prisms parallel to the direction of light propagation inthe LGP, a low index film (ULI) refractive index (RI) of 1.20, and anLGP w/o extractors.

With the same construction as Example 8 except that the low index filmRI is increased to 1.2, most of light emitted by the LEDs and enteringthe LGP from the left edge is guided between the LGP bottom and LGP/lowindex interface by TIR; about 5% of light leaks through LGP/low indexfilm interface and be reflected back again by the top surface of thelight redirecting film with TIR, as shown in FIG. 19.

Example 11 (Modeling)

The low index film refractive index (ULI RI) vs. light transport withand without the light redirecting prism film was determined and a familyof curves for different LGP refractive indices. Simulations were used topredict the impact of low index film RI and LGP RI on light transport inthe optical constructions.

An optical construction similar to that of Example 5 was modeled exceptthat the light redirecting prism film was removed. Low index film RI wasvaried from 1.0 (air) to 1.5; LGP RI was varied from 1.49 (PMMA) to 1.60(Polycarbonate). The amount of light guided between LGP/low index filminterface and LGP bottom was plotted in FIG. 20A.

Three points, A, B, and C, are shown indicated FIG. 20A. If the LGP ismade of PMMA with RI of 1.49 and combined with a low index film havingan RI of 1.25, then approximately 90% of total light is TIR guided andthe other 10% is not, as shown by point A. If the LGP is made ofPolycarbonate with an RI of 1.60, then 100% of light can be TIR guidedeven with a low index film having an RI of 1.25, as shown by point B.For an LGP made of PMMA (RI 1.49), the low index film RI has to be aslow as 1.10 in order for all light to be TIR guided, as shown by pointC.

An optical construction similar to Example 5 (including the lightredirecting film) was modeled. The low index film refractive index (ULIRI) was varied from 1.0 (air) to 1.5. The LGP RI was varied from 1.49(PMMA) to 1.6 (Polycarbonate). The amount of light guided between thetop surface of the light redirecting film and the LGP bottom was plottedin FIG. 20B. Compared with the results of FIG. 20A, this example showsthat light redirecting films having prisms that run perpendicular to thedirection of light propagation in the LGP has little effect on theamount of TIR guided light.

An optical construction similar to Example 8 was modeled. The low indexfilm refractive index (ULI RI) was varied from 1.0 (air) to 1.5; the LGPRI was varied from 1.49 (PMMA) to 1.60 (Polycarbonate). The amount oflight guided between top surface of prism and LGP bottom was plotted inFIG. 20C. This example shows that with a light redirecting film havingprisms running parallel to the direction of light propagation in theLGP, most light can be TIR guided, relatively independent of low indexfilm RI and LGP RI. However, the proportion of light guided at theLGP/low index film interface vs. the prism/air interface depends on LGPRI and low index film RI, as shown in FIG. 19A.

Example 12 (Modeling)

The low index optical layer between low absorption region (LGP) and highabsorption region (top films) reduces exposure of most of the light tothe higher absorption layer. MOFs generally have higher absorption thanan LGP; therefore it is desirable to reduce the exposure of the light tothe MOF until the light is extracted, which causes the light to take ashort traverse through the MOF at an angle close to surface normal,maximizes system efficiency. This example demonstrates how much lowindex film RI can affect system efficiency when a high absorption regionis attached to a LGP.

The simulated structure is shown in FIG. 21A. LEDs with Lambertianemission profile and height of 3 mm were placed closely to theilluminated edge of a LGP with the RI of 1.49. The LGP was 6 mm thick,90 mm wide, and 300 mm long (from illuminated edge to the distal edge).An absorption region, used to simulating optical films such as DBEF-Q,was placed above the LGP with a low index film (ULI) in between. Theabsorption region was 0.05 mm thick, with the transmission of 95% permm, and evenly splits the incident light (50% reflection and 50%transmission.) Circular dots with the diameter of 0.75 mm and lambertianreflectance of 100% were placed on the bottom surface of LGP for lightextraction.

Bezier placement, provided by LightTools 6.0, was used to vary the dotsdensity in order to achieve reasonable spatial uniformity. The front andback edges of the LGP were set as perfect mirrors. A white backreflector was placed under the LGP; the reflector has lambertianreflectance of 100%. Extraction efficiency and absorption by the highabsorption region were first calculated using the 300 mm long LGP andthen scaled up for 32″ (16″×27.7″) and 52″ diagonal (26″×45″)backlights. The amount of light being absorbed by the high absorptionregion for 32″ and 52″ backlights was plotted against the low index RIin FIG. 21B. For each size, two scenarios with light injected from longedge or short edge were both calculated and plotted. For a 52″ TV withLEDs arranged on the short edges (typically left and right edge), theabsorption by the high absorption region would be 55% if an opticallayer with an RI of 1.47 were used, as shown by point A. The absorptionwould be only 14% if a low index layer with RI of 1.2 were used, asshown by point B.

Item 1 is an optical construction, comprising:

a light guide having first surface and a second surface comprising amajor light exit surface of the light guide;

a light redirecting film; and

a low index layer disposed between the light guide and the lightredirecting film, the low index layer having an index of refraction notgreater than 1.35, the low index layer attached to the second surface ofthe light guide and to the light redirecting film.

Item 2 is the optical construction of item 1, wherein the first surfaceof the low index layer is attached to the major light exit surface ofthe light guide by an adhesive layer.

Item 3 is the optical construction of item 1, wherein the first surfaceof low index layer is attached to the major light exit surface of thelight guide by formation of the low index layer on the major light exitsurface of the light guide.

Item 4 is the optical construction of item 1, wherein the low indexlayer is attached to the major light exit surface of the light guidethrough one or more intervening layers disposed between the low indexlayer and the light guide.

Item 5 is the optical construction of item 1, wherein the low indexlayer is attached to the light redirecting film by an adhesive layer.

Item 6 is the optical construction of item 1, wherein the low indexlayer is attached to the light redirecting film by formation of the lowindex layer on the light redirecting film.

Item 7 is the optical construction of item 1, wherein the low indexlayer is attached to the light redirecting film through one or moreintervening layers disposed between the low index layer and the lightguide.

Item 8 is the optical construction of item 7, wherein the one or moreintervening layers comprises a high absorption layer.

Item 9 is the optical construction of item 7, wherein the one or moreintervening layers comprises a polarizer.

Item 10 is the optical construction of item 7, wherein the one or moreintervening layers comprises a diffuser.

Item 11 is the optical construction of item 10, wherein the diffusercomprises a plurality of voids.

Item 12 is the optical construction of item 1, wherein the lightredirecting film comprises a first surface and a second surface havingstructured features.

Item 13 is the optical construction of item 12, wherein the secondsurface of the light redirecting film is oriented away from the lightguide.

Item 14 is the optical construction of item 12, wherein the secondsurface of the light redirecting film is oriented toward the lightguide.

Item 15 is the optical construction of item 1, wherein: the light guidehas an absorption A1; and the light redirecting film has an absorptiongreater than A1.

Item 16 is the optical construction of item 1, further comprising a highabsorption layer disposed between the low index layer and the lightredirecting film, the high absorption layer having an absorption greaterthan an absorption of the light guide.

Item 17 is the optical construction of item 1, wherein the lightredirecting film has an absorption greater than an absorption of thelight guide.

Item 18 is the optical construction of item 1, wherein the low indexlayer is repositionable.

Item 19 is an optical construction, comprising: a low index layer havingan index of refraction, Nuli, where Nuli is not greater than about 1.35;

a high absorption layer; and

a light redirecting film, wherein substantial portions of each of twoneighboring films in the optical construction are in physical contactwith each other.

Item 20 is the optical construction of item 19, wherein the low indexlayer has an index of refraction that is not greater than about 1.2.

Item 21 is the optical construction of item 19, wherein the low indexlayer has an index of refraction that is not greater than about 1.1.

Item 22 is the optical construction of item 19, wherein the low indexlayer has a haze that is less than about 5%.

Item 23 is the optical construction of item 19, wherein the highabsorption layer comprises a multilayer optical film.

Item 24 is the optical construction of item 19, wherein the thickness ofthe low index layer is not less than about 2 microns.

Item 25 is the optical construction of item 19, wherein the thickness ofthe low index layer is not less than about 1 micron.

Item 26 is the optical construction of item 19, wherein the low indexlayer comprises a plurality of voids.

Item 27 is the optical construction of item 26, wherein the voids areinterconnected.

Item 28 is the optical construction of item 26, wherein a volumefraction of the plurality of voids is not less than about 20%.

Item 29 is the optical construction of item 26, wherein a volumefraction of the plurality of voids is not less than about 40%.

Item 30 is the optical construction of item 26, wherein a local volumefraction of the plurality of voids varies along a thickness direction ofthe low index layer.

Item 31 is the optical construction of item 26, wherein a first localvolume fraction of the plurality of voids at a first surface of the lowindex layer is greater than a second local volume fraction of theplurality of voids at a second surface of the low index layer.

Item 32 is the optical construction of item 26, wherein the low indexlayer further comprises:

a binder; and

a plurality of particles wherein a weight ratio of the binder to theplurality of voids is not less than about 1:2.

Item 33 is the optical construction of item 32, wherein the plurality ofparticles has an average size of less than about 100 nm.

Item 34 is the optical construction of item 19, wherein more than 50% ofeach of two neighboring films in the optical construction are inphysical contact with each other.

Item 35 is the optical construction of item 19, wherein the highabsorption layer comprises a reflective polarizer.

Item 36 is an optical construction, comprising:

at least one light guide having a first surface, a second surface, andan index of refraction N1, the second surface being a major light exitsurface of the light guide;

a low index layer having an index of refraction, Nuli, where Nuli isless than N1; and

a light redirecting film, wherein substantial portions of each of twoneighboring films in the optical construction are in physical contactwith each other.

Item 37 is the optical construction of item 36, wherein the low indexlayer increases optical confinement of the light propagated in the lightguide.

Item 38 is the optical construction of item 36, further comprising ahigh absorption layer between the low index layer and the lightredirecting film, wherein the high absorption layer has an absorptionthat is about twice an absorption of the light guide.

Item 39 is the optical construction of item 38, wherein:

the light guide includes light extraction features; and

the low index layer is configured to reduce the amount of light enteringthe high absorption layer until the light is extracted by a lightextraction feature in comparison to an optical construction without thelow index layer.

Item 40 is the optical construction of item 38, wherein the highabsorption layer comprises a reflective polarizer.

Item 41 is the optical construction of item 36, wherein the lightredirecting film comprises a first surface and a second surface, thesecond surface of the light directing film having refractive structuresdisposed thereon, wherein the second surface of the light redirectingfilm is disposed away from the light guide.

Item 42 is the optical construction of item 36, wherein the lightredirecting film comprises a first surface and a second surface, thesecond surface of the light directing film having refractive structuresdisposed thereon, wherein the second surface of the light redirectingfilm is disposed toward the light guide, the low index layer at leastpartially fills spaces between the refractive structures.

Item 43 is the optical construction of item 36, wherein the low indexlayer is attached to the major exit surface of the light guide with anadhesive.

Item 44 is the optical construction of item 36, wherein the light guideincludes extraction features on the first surface of the light guide.

Item 45 is the optical construction of item 36, wherein:

the light guide transports light along a light transport axis; and

the light redirecting film comprises refractive structures which areoriented substantially parallel to the light transport axis.

Item 46 is the optical construction of item 45, wherein the refractivestructures comprise one or more of prisms, linear prisms, piece-wiselinear prisms, and lenticular structures.

Item 47 is the optical construction of item 36, further comprising ahigh absorption layer, wherein the light guide has an absorption A1 andthe high absorption layer has an absorption A2, where A2 is greater thanA1.

Item 48 is the optical construction of item 36, wherein the light guidecomprises multiple light guides.

Item 49 is the optical construction of item 48, wherein the multiplelight guides are tiled.

Item 50 is the optical construction of item 48, wherein the multiplelight guides are arranged in an array.

Item 51 is the optical construction of item 48, further comprisingmultiple light sources respectively associated with the multiple lightguides.

Item 52 is the optical construction of item 48, further comprisingmultiple light sources, wherein light outputs from the multiple lightsources are individually controllable.

Item 53 is the optical construction of item 48, further comprisingmultiple light sources respectively associated with the multiple lightguides, wherein a light source associated with a particular light guideis positioned under an adjacent light guide.

Item 54 is the optical construction of item 36, further comprising adown converting element.

Item 55 is the optical construction of item 36, wherein the light guidehas input region comprising one or more of a flat input edge, astructured surface, and a channel.

Item 56 is the optical construction of item 36, further comprising lightextraction features disposed on the first surface of the light guide.

Item 57 is the optical construction of item 36, wherein the low indexlayer is configured to reflect light exiting from the light guide at afirst set of exit angles and the light re-directing film is configuredto reflect light exiting from the light guide at a second set of exitangles.

Item 58 is the optical construction of item 36, further comprising oneor more of a specular reflector, a semi-specular reflector, an enhancedspecular reflector, an extended-band enhanced specular reflector, and adiffuse reflector.

Item 59 is the optical construction of item 36, wherein N1 is about1.49.

Item 60 is the optical construction of item 36, wherein Nuli is in arange of about 1.10 to about 1.35.

Item 61 is the optical construction of item 36, wherein the low indexlayer has a haze that is not greater than 1%.

Item 62 is the optical construction of item 36, wherein the low indexlayer has a haze up to about 10%.

Item 63 is the optical construction of item 36, wherein the low indexlayer comprises gel or fumed silica.

Item 64 is the optical construction of item 36, wherein the thickness ofthe low index layer is not less than about 2 microns.

Item 65 is the optical construction of item 36, wherein the thickness ofthe low index layer is not less than about 1 micron.

Item 66 is the optical construction of item 36, wherein the low indexlayer comprises a plurality of voids.

Item 67 is the optical construction of item 66, wherein the voids areinterconnected.

Item 68 is the optical construction of item 66, wherein a volumefraction of the plurality of voids is not less than about 20%.

Item 69 is the optical construction of item 66, wherein a volumefraction of the plurality of voids is not less than about 40%.

Item 70 is the optical construction of item 66, wherein a local volumefraction of the plurality of voids varies along a thickness direction ofthe low index layer.

Item 71 is the optical construction of item 66, wherein a first localvolume fraction of the plurality of voids at a first surface of the lowindex layer is greater than a second local volume fraction of theplurality of voids at a second surface of the low index layer.

Item 72 is the optical construction of item 66, wherein the low indexlayer further comprises:

a binder; and

a plurality of particles wherein a weight ratio of the binder to theplurality of voids is not less than about 1:2.

Item 73 is the optical construction of item 36, wherein a polarizer isdisposed between the light redirecting film and the low index layer.

Item 74 is the optical construction of item 73, wherein the polarizercomprises a multilayer optical film (MOF).

Item 75 is the optical constriction of item 74, wherein the MOFcomprises an extended band reflective polarizer.

Item 76 is the optical construction of item 36, wherein the lightre-directing film comprises a first set of prisms having a first heightand second set of prisms having a second height different from the firstheight.

Item 77 is the optical construction of item 36, wherein the lightre-directing film comprises one or more linear or piece-wise linearprisms that have a variable height along a length of the one or moreprisms.

Item 78 is the optical construction of item 36, wherein the lightre-directing film has an index of refraction in a range of about 1.5 toabout 1.8.

Item 79 is an optical construction, comprising:

-   -   a light guide, having first and second major surfaces and an        index of refraction N1;

a low index layer having first and second major surfaces, the low indexlayer having an index of refraction, Nuli, where Nuli is less than N1, asubstantial portion of the first major surface of the low index layer inphysical contact with the second major surface of the light guide;

a high absorption layer having a first major surface and a second majorsurface, a substantial portion of the first major surface of the highabsorption layer in physical contact with the second major surface ofthe low index layer; and

a prism film, having a first major surface and a second major surface,the first major surface comprising linear prisms and a substantialportion of the first major surface of the light re-directing film inphysical contact with the second major surface of the high absorptionlayer, wherein the low index layer reflects light exiting from the lightguide at a first set of exit angles and the light re-directing film isconfigured to reflect light exiting from the light guide at a second setof exit angles.

All patents, patent applications, and other publications cited above areincorporated by reference into this document as if reproduced in full.While specific embodiments and examples of the invention are describedin detail above to facilitate explanation of various aspects of theinvention, it should be understood that the intention is not to limitthe invention to the specifics of the examples. Rather, the intention isto cover all modifications, embodiments, and alternatives falling withinthe scope of the invention as defined by the appended claims.

What is claimed is:
 1. An optical construction, comprising: a lightguide having first surface and a second surface comprising a major lightexit surface of the light guide; a light redirecting film; and a lowindex layer disposed between the light guide and the light redirectingfilm, the low index layer having an index of refraction not greater than1.35, the low index layer attached to the second surface of the lightguide and to the light redirecting film; wherein in the lightredirecting film comprises structured features.
 2. The opticalconstruction of claim 1, wherein the structured features include prisms.3. The optical construction of claim 2, wherein the prisms includelinear prisms.
 4. The optical construction of claim 2, wherein theprisms are oriented parallel to a direction of light propagation in thelight guide.
 5. The optical construction of claim 1, wherein the lightredirecting film has an absorption higher than an absorption of thelight guide, and the light redirecting film includes scatteringmaterials.
 6. The optical construction of claim 1, wherein thestructured features include lenticulars.
 7. The optical construction ofclaim 1, wherein the structured features are continuous.
 8. The opticalconstruction of claim 1, wherein the structured features are piecewisecontinuous.
 9. The optical construction of claim 1, wherein thestructured features have in-plane serpentine variations.
 10. The opticalconstruction of claim 1, wherein the structured features includevariations in a peak height along the same structured feature.
 11. Theoptical construction of claim 1, wherein the structured features includevariations in peak height between different structured features.
 12. Theoptical construction of claim 1, further comprising a phosphor disposednear a light entry region or light exit region of the light guide. 13.The optical construction of claim 1, wherein the low index layer has agradient of at least one of index of refraction and porosity.
 14. Theoptical construction of claim 1, further comprising one or more lightsources oriented near an input edge of the light guide.
 15. The opticalconstruction of claim 14, wherein the one or more light sources includeone or more LEDs.
 16. The optical construction of claim 15, wherein theone or more LEDs include LEDs that emit red, green, or blue light. 17.The optical construction of claim 15, further comprising one or morelight sensors and a control system, wherein the one or more lightsensors are configured to be used with the control system toindependently control the light emitted from the one or more LEDs.
 18. Asign, comprising the optical system of claim
 1. 19. A luminaire,comprising the optical system of claim
 1. 20. A display, comprising theoptical system of claim
 1. 21. An optical construction, comprising: alight guide having first surface and a second surface comprising a majorlight exit surface of the light guide; a reflective polarizer; and a lowindex layer disposed between the light guide and the reflectivepolarizer, the low index layer having an index of refraction not greaterthan 1.35, the low index layer attached to the second surface of thelight guide and to the reflective polarizer.
 22. The opticalconstruction of claim 21, wherein the reflective polarizer is acollimating reflective polarizer.